[ReflectionContext] Rewrite the Mach reader path.
[ReflectionContext] Rewrite the Mach reader path.

There are several problems fixed by this commit.
-> It removes the dependency on a system header.
-> It works when the pointer size for host and target don't match
(i.e. 32-bits/64-bits and viceversa)
-> It reads the file lazily, mapping only the sections that are
needed. The old approach mapped the whole __TEXT segment, causing
significant slowdowns when trying to integrate this API in the
debugger. Now it's almost instant (10 seconds vs milliseconds).

<rdar://problem/42306551>

tkremenek/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,gribozavr/swift,amraboelela/swift,atrick/swift,benlangmuir/swift,allevato/swift,austinzheng/swift,jckarter/swift,xwu/swift,airspeedswift/swift,parkera/swift,shajrawi/swift,brentdax/swift,nathawes/swift,ahoppen/swift,gribozavr/swift,apple/swift,xedin/swift,danielmartin/swift,atrick/swift,lorentey/swift,xwu/swift,brentdax/swift,atrick/swift,practicalswift/swift,aschwaighofer/swift,jckarter/swift,shahmishal/swift,JGiola/swift,sschiau/swift,harlanhaskins/swift,CodaFi/swift,jmgc/swift,alblue/swift,lorentey/swift,hooman/swift,austinzheng/swift,shahmishal/swift,JGiola/swift,atrick/swift,danielmartin/swift,parkera/swift,jopamer/swift,aschwaighofer/swift,hooman/swift,devincoughlin/swift,parkera/swift,xedin/swift,airspeedswift/swift,nathawes/swift,nathawes/swift,jopamer/swift,amraboelela/swift,practicalswift/swift,stephentyrone/swift,xedin/swift,devincoughlin/swift,xedin/swift,apple/swift,sschiau/swift,jckarter/swift,apple/swift,CodaFi/swift,aschwaighofer/swift,jmgc/swift,jmgc/swift,austinzheng/swift,glessard/swift,gribozavr/swift,gregomni/swift,allevato/swift,gribozavr/swift,practicalswift/swift,amraboelela/swift,natecook1000/swift,gregomni/swift,lorentey/swift,xedin/swift,brentdax/swift,danielmartin/swift,shajrawi/swift,shajrawi/swift,alblue/swift,karwa/swift,jopamer/swift,parkera/swift,karwa/swift,stephentyrone/swift,alblue/swift,benlangmuir/swift,danielmartin/swift,gribozavr/swift,xedin/swift,apple/swift,roambotics/swift,apple/swift,harlanhaskins/swift,hooman/swift,shahmishal/swift,airspeedswift/swift,lorentey/swift,roambotics/swift,gregomni/swift,gregomni/swift,hooman/swift,sschiau/swift,amraboelela/swift,jckarter/swift,stephentyrone/swift,alblue/swift,jckarter/swift,allevato/swift,jopamer/swift,shahmishal/swift,allevato/swift,tkremenek/swift,JGiola/swift,karwa/swift,jmgc/swift,jopamer/swift,shahmishal/swift,airspeedswift/swift,xedin/swift,devincoughlin/swift,devincoughlin/swift,lorentey/swift,devincoughlin/swift,shahmishal/swift,rudkx/swift,allevato/swift,ahoppen/swift,brentdax/swift,lorentey/swift,lorentey/swift,shajrawi/swift,devincoughlin/swift,CodaFi/swift,amraboelela/swift,danielmartin/swift,lorentey/swift,practicalswift/swift,austinzheng/swift,jmgc/swift,benlangmuir/swift,jckarter/swift,glessard/swift,nathawes/swift,stephentyrone/swift,hooman/swift,CodaFi/swift,airspeedswift/swift,CodaFi/swift,tkremenek/swift,shajrawi/swift,sschiau/swift,austinzheng/swift,aschwaighofer/swift,karwa/swift,jckarter/swift,shajrawi/swift,atrick/swift,devincoughlin/swift,parkera/swift,natecook1000/swift,natecook1000/swift,stephentyrone/swift,amraboelela/swift,shahmishal/swift,parkera/swift,sschiau/swift,parkera/swift,stephentyrone/swift,danielmartin/swift,xwu/swift,rudkx/swift,xwu/swift,roambotics/swift,danielmartin/swift,amraboelela/swift,tkremenek/swift,karwa/swift,harlanhaskins/swift,CodaFi/swift,JGiola/swift,benlangmuir/swift,ahoppen/swift,karwa/swift,practicalswift/swift,xedin/swift,parkera/swift,roambotics/swift,karwa/swift,alblue/swift,sschiau/swift,roambotics/swift,gribozavr/swift,harlanhaskins/swift,brentdax/swift,sschiau/swift,rudkx/swift,natecook1000/swift,glessard/swift,nathawes/swift,harlanhaskins/swift,natecook1000/swift,natecook1000/swift,CodaFi/swift,roambotics/swift,sschiau/swift,gregomni/swift,jmgc/swift,tkremenek/swift,natecook1000/swift,glessard/swift,gribozavr/swift,tkremenek/swift,harlanhaskins/swift,shajrawi/swift,aschwaighofer/swift,xwu/swift,jopamer/swift,shahmishal/swift,practicalswift/swift,airspeedswift/swift,alblue/swift,glessard/swift,aschwaighofer/swift,austinzheng/swift,atrick/swift,brentdax/swift,jopamer/swift,airspeedswift/swift,alblue/swift,austinzheng/swift,nathawes/swift,nathawes/swift,hooman/swift,jmgc/swift,glessard/swift,gribozavr/swift,apple/swift,harlanhaskins/swift,devincoughlin/swift,hooman/swift,ahoppen/swift,xwu/swift,shajrawi/swift,benlangmuir/swift,JGiola/swift,xwu/swift,gregomni/swift,rudkx/swift,rudkx/swift,practicalswift/swift,stephentyrone/swift,allevato/swift,karwa/swift,tkremenek/swift,JGiola/swift,brentdax/swift,aschwaighofer/swift,allevato/swift,practicalswift/swift,benlangmuir/swift