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

[sharadhr]

STL/stl/modules/std.ixx

Lines 39 to 40 in cbe2ee9

	#pragma warning(push)
	#pragma warning(disable : 5244) // '#include <meow>' in the purview of module 'std' appears erroneous.

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.

[StephanTLavavej]

I have not yet attempted to validate our modules with Clang, but I agree that suppressions like this will be necessary. We detect __clang__ all the time (search the codebase for it).

I would really like for clang-cl to support modules (last I heard, it wasn't supported for what I considered silly reasons; I am unsure of the current status).

[sharadhr]

silly reasons

I couldn't have put it better. It sort-of supports modules, but some options to configure the PCM output need to be configured by switching the driver mode back to clang. This thread summarises the situation.