[CodeGen] Emit a CoreFoundation link guard when @available is used

After r297760, __isOSVersionAtLeast in compiler-rt loads the CoreFoundation
symbols at runtime. This means that `@available` will always fail when used in a
binary without a linked CoreFoundation.

This commit forces Clang to emit a reference to a CoreFoundation symbol when
`@available` is used to ensure that linking will fail when CoreFoundation isn't
linked with the build product.

rdar://31039592

Differential Revision: https://reviews.llvm.org/D30977

llvm-svn: 298588