SIL: support non-clang compilers again
SIL: support non-clang compilers again

The toolchain has traditionally been buildable with clang, cl (MSVC),
and gcc.  `__has_feature` is a clang-specific directive, ensure that we
are portable to other compilers by providing a definition of the macro
incase it is not available (which also includes older clang releases).

benlangmuir/swift,rudkx/swift,gregomni/swift,rudkx/swift,gregomni/swift,apple/swift,atrick/swift,glessard/swift,apple/swift,glessard/swift,ahoppen/swift,roambotics/swift,gregomni/swift,roambotics/swift,roambotics/swift,apple/swift,atrick/swift,JGiola/swift,JGiola/swift,glessard/swift,apple/swift,atrick/swift,atrick/swift,apple/swift,benlangmuir/swift,roambotics/swift,rudkx/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,ahoppen/swift,atrick/swift,glessard/swift,JGiola/swift,ahoppen/swift,ahoppen/swift,roambotics/swift,JGiola/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,rudkx/swift,rudkx/swift,JGiola/swift,apple/swift,glessard/swift,rudkx/swift,gregomni/swift,gregomni/swift,ahoppen/swift,atrick/swift,gregomni/swift,JGiola/swift