[CMake] Default to Debug builds
[CMake] Default to Debug builds

When CMake is invoked manually to configure a Swift build, it fails
because no default value is specified for the build type:

    CMake Error at cmake/modules/SwiftUtils.cmake:94 (message):
      Unknown build type:
    Call Stack (most recent call first):
      CMakeLists.txt:98 (is_build_type_optimized)

Instead we take a cue from LLVM, and specify a default value for
CMAKE_BUILD_TYPE.  By providing a default, we reduce the number of
mandatory CMake configuration parameters, and make it easier for
people to get started.

The conditional was taken from llvm/CMakeLists.txt.  The check for
CMAKE_CONFIGURATION_TYPES is necessary to account for
multi-configuration generators [1].

The default was taken to be "Debug", simply because it was the one
which LLVM used.

[1]: https://cmake.org/pipermail/cmake/2006-February/008371.html

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