Add a Clang-specific diagnostic ignore push-pop for module std; #5975

It seems the MSVC STL module works with clang++.exe (this is the driver with the GNU-like command-line interface, but targets the MSVC ABI). However, the module needs to have additional guards for Clang so that the same warnings of having #include <angle> and defining module std are suppressed. The core of it is:

#if defined(__clang__)
    #pragma clang diagnostic push
    #pragma clang diagnostic ignored "-Winclude-angled-in-module-purview"
    #pragma clang diagnostic ignored "-Wreserved-module-identifier"
#endif

...

#if defined(__clang__)
    #pragma clang diagnostic pop
#endif

But I suspect we need to be much more judicious about the macro guards, because clang-cl.exe disguises itself as MSVC to source code, with the exception that __clang__ is also defined.