[clang] Require including config.h for CLANG_DEFAULT_STD_C

This makes CLANG_DEFAULT_STD_C(XX) always be defined, defaulting to
lang_unspecified, so you are forced to check its value instead of using
an #ifdef. This should help avoid accidentally omitting the include in
places where that's important, so that the default language version bug
isn't re-introduced.

Reviewed By: hokein, dexonsmith

Differential Revision: https://reviews.llvm.org/D124974