[compiler-rt][builtins] __isOSVersionAtLeast should load CoreFoundation

symbols dynamically

The CoreFoundation symbols uses by __isOSVersionAtLeast should be loaded at
runtime to ensure that the programs that don't use @available won't have to be
linked to CoreFoundation.

The Clang frontend IRGen library will need to emit a CoreFoundation symbol
when @available is used to ensure that programs that actually use @available
are linked to CoreFoundation.

rdar://31039554

llvm-svn: 297760