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

https://github.com/microsoft/STL/blob/cbe2ee99caf122555a305d18f69e57c75b3fe5ec/stl/modules/std.ixx#L39-L40

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.

Start at stl/modules/std.ixx around lines 39–40 and compare how clang++.exe and clang-cl.exe define compiler macros. Reproduce the module warnings for the angle-bracket include and module name, then determine guards that suppress them for Clang without changing MSVC behavior; done means both diagnostics are appropriately scoped and the warning-free module remains compatible with both drivers.