Guard use of custom attributes against macroization

[CaseyCarter]

Define _MSVC_MEOW macro in <yvals_core.h>for each [[msvc::meow]] custom attribute with surrounding push_macro/pop_macro guards.

Fixes #2645

[CaseyCarter]

One of the compiler devs is reworking how the constexpr evaluator interfaces with construct_at / ranges::construct_at and may be adding a custom attribute. If so, I'll probably integrate this PR into their changes so the same "guard" mechanism is used for the new attribute.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Macros incoming! Shields up, red alert! 🚨 🛡️ 😹

[MikeGitb]

@CaseyCarter : Are those [[msvc::meow]] attributes documented somewhere? Or, more to the point: Are they officially supported or just something for toolchain implementors?

All I found was https://docs.microsoft.com/en-us/cpp/cpp/attributes?view=msvc-170, which only mentions standard attributes and gsl::suppress(rules)?

[CaseyCarter]

@CaseyCarter : Are those [[msvc::meow]] attributes documented somewhere? Or, more to the point: Are they officially supported or just something for toolchain implementors?

[[msvc::no_unique_address]] should be documented, but [[msvc::known_semantics]], [[msvc::noop_dtor]], and [[msvc::constexpr]] are purely for internal communication between the STL and C1XX.