[clang][Headers] Avoid compiler warnings in builtin headers

[clang][Headers] Avoid compiler warnings in builtin headers

While debugging module support using -Wsystem-headers, we discovered that if
-Werror, and -Wundef or -Wmacro-redefined are specified, they can cause errors
to be generated in these builtin headers.

Differential Revision: https://reviews.llvm.org/D130800