WG14 N3469 changed _Lengthof to _Countof but it also introduced the
<stdcountof.h> header to expose a macro with a non-ugly identifier. GCC
vends this header as part of the compiler implementation, so Clang
should do the same.
Suggested-by: Alejandro Colomar <alx@kernel.org>
All of the _Builtin_stdarg and _Builtin_stddef submodules need to be
allowed from [no_undeclared_includes] modules. Split the builtin headers
tests out from the compiler_builtins test so that the testing modules
can be modified without affecting the other many tests that use
Inputs/System/usr/include.