[Modules] no_undeclared_includes modules (Apple Darwin) don't work the clang modules (#68241)

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.