[build-script] Provide centralized interface for command execution in build-script-impl
[build-script] Provide centralized interface for command execution in build-script-impl

Abstract `set -x; ... { set +x; }` block.
As a side effect, prevent displaying confusing trace like:
```
++ cmake_config_opt cmark
++ product=cmark
++ [[ Ninja == \X\c\o\d\e ]]
```

tjw/swift,IngmarStein/swift,tinysun212/swift-windows,rudkx/swift,allevato/swift,benlangmuir/swift,IngmarStein/swift,kperryua/swift,harlanhaskins/swift,IngmarStein/swift,zisko/swift,gribozavr/swift,gmilos/swift,xwu/swift,ahoppen/swift,swiftix/swift,xedin/swift,amraboelela/swift,sschiau/swift,CodaFi/swift,shajrawi/swift,swiftix/swift,gmilos/swift,airspeedswift/swift,hooman/swift,modocache/swift,lorentey/swift,felix91gr/swift,practicalswift/swift,alblue/swift,gmilos/swift,gottesmm/swift,shahmishal/swift,shahmishal/swift,hughbe/swift,jmgc/swift,ahoppen/swift,calebd/swift,amraboelela/swift,shajrawi/swift,sschiau/swift,stephentyrone/swift,zisko/swift,sschiau/swift,swiftix/swift,brentdax/swift,alblue/swift,parkera/swift,codestergit/swift,tinysun212/swift-windows,uasys/swift,natecook1000/swift,ahoppen/swift,jmgc/swift,calebd/swift,CodaFi/swift,huonw/swift,swiftix/swift,karwa/swift,karwa/swift,practicalswift/swift,austinzheng/swift,ben-ng/swift,rudkx/swift,arvedviehweger/swift,jckarter/swift,amraboelela/swift,atrick/swift,arvedviehweger/swift,danielmartin/swift,milseman/swift,hooman/swift,russbishop/swift,shahmishal/swift,stephentyrone/swift,austinzheng/swift,deyton/swift,devincoughlin/swift,arvedviehweger/swift,karwa/swift,rudkx/swift,kperryua/swift,frootloops/swift,deyton/swift,calebd/swift,jckarter/swift,devincoughlin/swift,deyton/swift,xwu/swift,OscarSwanros/swift,zisko/swift,shajrawi/swift,calebd/swift,harlanhaskins/swift,tardieu/swift,apple/swift,apple/swift,gottesmm/swift,CodaFi/swift,deyton/swift,hooman/swift,tardieu/swift,modocache/swift,apple/swift,deyton/swift,ben-ng/swift,harlanhaskins/swift,danielmartin/swift,nathawes/swift,therealbnut/swift,xwu/swift,shajrawi/swift,JaSpa/swift,airspeedswift/swift,practicalswift/swift,Jnosh/swift,atrick/swift,frootloops/swift,russbishop/swift,zisko/swift,aschwaighofer/swift,alblue/swift,jckarter/swift,danielmartin/swift,manavgabhawala/swift,therealbnut/swift,hughbe/swift,swiftix/swift,Jnosh/swift,arvedviehweger/swift,stephentyrone/swift,tardieu/swift,milseman/swift,manavgabhawala/swift,ahoppen/swift,austinzheng/swift,ben-ng/swift,natecook1000/swift,xedin/swift,calebd/swift,shahmishal/swift,bitjammer/swift,gmilos/swift,gribozavr/swift,xedin/swift,arvedviehweger/swift,zisko/swift,JGiola/swift,jtbandes/swift,modocache/swift,gmilos/swift,tjw/swift,xedin/swift,karwa/swift,allevato/swift,gregomni/swift,calebd/swift,codestergit/swift,atrick/swift,tkremenek/swift,gribozavr/swift,benlangmuir/swift,benlangmuir/swift,parkera/swift,parkera/swift,xedin/swift,brentdax/swift,gregomni/swift,parkera/swift,airspeedswift/swift,IngmarStein/swift,modocache/swift,deyton/swift,xwu/swift,tkremenek/swift,stephentyrone/swift,Jnosh/swift,gottesmm/swift,devincoughlin/swift,hooman/swift,modocache/swift,return/swift,huonw/swift,dreamsxin/swift,JaSpa/swift,manavgabhawala/swift,aschwaighofer/swift,jmgc/swift,hughbe/swift,harlanhaskins/swift,return/swift,harlanhaskins/swift,amraboelela/swift,jmgc/swift,apple/swift,gregomni/swift,JaSpa/swift,felix91gr/swift,gribozavr/swift,milseman/swift,frootloops/swift,gottesmm/swift,tardieu/swift,practicalswift/swift,amraboelela/swift,shahmishal/swift,zisko/swift,felix91gr/swift,kstaring/swift,gribozavr/swift,tjw/swift,amraboelela/swift,tardieu/swift,benlangmuir/swift,allevato/swift,benlangmuir/swift,alblue/swift,Jnosh/swift,practicalswift/swift,Jnosh/swift,apple/swift,kperryua/swift,alblue/swift,russbishop/swift,brentdax/swift,airspeedswift/swift,jckarter/swift,kperryua/swift,djwbrown/swift,roambotics/swift,huonw/swift,gribozavr/swift,manavgabhawala/swift,JaSpa/swift,shajrawi/swift,brentdax/swift,JaSpa/swift,hughbe/swift,atrick/swift,uasys/swift,jmgc/swift,parkera/swift,jtbandes/swift,djwbrown/swift,arvedviehweger/swift,gottesmm/swift,glessard/swift,OscarSwanros/swift,roambotics/swift,stephentyrone/swift,xwu/swift,swiftix/swift,frootloops/swift,harlanhaskins/swift,rudkx/swift,austinzheng/swift,gmilos/swift,brentdax/swift,jtbandes/swift,therealbnut/swift,russbishop/swift,tinysun212/swift-windows,jopamer/swift,ahoppen/swift,OscarSwanros/swift,therealbnut/swift,kperryua/swift,JGiola/swift,aschwaighofer/swift,JaSpa/swift,manavgabhawala/swift,ahoppen/swift,tinysun212/swift-windows,lorentey/swift,jtbandes/swift,nathawes/swift,benlangmuir/swift,tjw/swift,uasys/swift,lorentey/swift,allevato/swift,parkera/swift,glessard/swift,calebd/swift,tjw/swift,tkremenek/swift,uasys/swift,brentdax/swift,tkremenek/swift,austinzheng/swift,return/swift,swiftix/swift,natecook1000/swift,milseman/swift,hooman/swift,manavgabhawala/swift,JGiola/swift,bitjammer/swift,djwbrown/swift,tinysun212/swift-windows,felix91gr/swift,rudkx/swift,hughbe/swift,roambotics/swift,codestergit/swift,OscarSwanros/swift,xwu/swift,natecook1000/swift,hughbe/swift,frootloops/swift,alblue/swift,bitjammer/swift,djwbrown/swift,JGiola/swift,therealbnut/swift,gregomni/swift,lorentey/swift,glessard/swift,kperryua/swift,tardieu/swift,roambotics/swift,gribozavr/swift,djwbrown/swift,devincoughlin/swift,kstaring/swift,danielmartin/swift,tjw/swift,jtbandes/swift,practicalswift/swift,shajrawi/swift,return/swift,danielmartin/swift,ben-ng/swift,nathawes/swift,hooman/swift,nathawes/swift,roambotics/swift,lorentey/swift,glessard/swift,lorentey/swift,frootloops/swift,danielmartin/swift,jopamer/swift,ben-ng/swift,jckarter/swift,hughbe/swift,ben-ng/swift,gmilos/swift,huonw/swift,djwbrown/swift,glessard/swift,devincoughlin/swift,jopamer/swift,frootloops/swift,IngmarStein/swift,jmgc/swift,rudkx/swift,huonw/swift,return/swift,jtbandes/swift,xwu/swift,gottesmm/swift,karwa/swift,parkera/swift,jckarter/swift,nathawes/swift,OscarSwanros/swift,Jnosh/swift,practicalswift/swift,allevato/swift,felix91gr/swift,atrick/swift,huonw/swift,tjw/swift,arvedviehweger/swift,jopamer/swift,russbishop/swift,nathawes/swift,modocache/swift,natecook1000/swift,lorentey/swift,xedin/swift,airspeedswift/swift,russbishop/swift,xedin/swift,sschiau/swift,gribozavr/swift,kstaring/swift,karwa/swift,tkremenek/swift,aschwaighofer/swift,tkremenek/swift,allevato/swift,atrick/swift,jopamer/swift,tkremenek/swift,CodaFi/swift,apple/swift,stephentyrone/swift,sschiau/swift,kstaring/swift,modocache/swift,jopamer/swift,bitjammer/swift,jopamer/swift,kstaring/swift,jtbandes/swift,therealbnut/swift,xedin/swift,OscarSwanros/swift,hooman/swift,shajrawi/swift,gottesmm/swift,shahmishal/swift,shahmishal/swift,milseman/swift,IngmarStein/swift,devincoughlin/swift,amraboelela/swift,danielmartin/swift,aschwaighofer/swift,bitjammer/swift,kperryua/swift,huonw/swift,gregomni/swift,kstaring/swift,karwa/swift,austinzheng/swift,felix91gr/swift,uasys/swift,airspeedswift/swift,codestergit/swift,tinysun212/swift-windows,devincoughlin/swift,harlanhaskins/swift,sschiau/swift,bitjammer/swift,airspeedswift/swift,felix91gr/swift,parkera/swift,shahmishal/swift,CodaFi/swift,lorentey/swift,codestergit/swift,practicalswift/swift,uasys/swift,sschiau/swift,austinzheng/swift,return/swift,codestergit/swift,IngmarStein/swift,CodaFi/swift,russbishop/swift,return/swift,shajrawi/swift,nathawes/swift,uasys/swift,kstaring/swift,ben-ng/swift,codestergit/swift,stephentyrone/swift,manavgabhawala/swift,karwa/swift,allevato/swift,roambotics/swift,devincoughlin/swift,aschwaighofer/swift,dreamsxin/swift,deyton/swift,natecook1000/swift,Jnosh/swift,alblue/swift,tardieu/swift,jmgc/swift,brentdax/swift,glessard/swift,OscarSwanros/swift,JGiola/swift,zisko/swift,JaSpa/swift,tinysun212/swift-windows,bitjammer/swift,djwbrown/swift,natecook1000/swift,jckarter/swift,sschiau/swift,milseman/swift,gregomni/swift,JGiola/swift,therealbnut/swift,CodaFi/swift,milseman/swift,aschwaighofer/swift