[libc] Define LLVM_LIBC_FUNCTION with a layer of macro expansion

Move the real LLVM_LIBC_FUNCTION macro definitions to
LLVM_LIBC_FUNCTION_IMPL and make LLVM_LIBC_FUNCTION a wrapper to
expand macros in its arguments.  This makes it possible to
compile libc implementation and test files with -Dfunc=othername.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146863