Don't reserve __builtin_types_compatible_p as a C++ keyword

Even g++ considers this a valid C++ identifier and it should only have been
visible in C mode.

Also drop the associated low-value diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197995 91177308-0d34-0410-b5e6-96231b3b80d8