`<yvals_core.h>`: Conformance issue on `[[msvc::known_semantics]]`

[frederick-vs-ja]

msvc and known_semantics are not reserved identifiers in the C++ Standard, so users are allowed to #define them to anything, which may break every header that directly uses [[msvc::known_semantics]] (e.g. <type_traits>).

IIUC the Standard equivalently requires that every non-Standard attribute used in a Standard header to be spelled in reserved identifiers, e.g. [[__msvc__::__known_semantics__]]. Does MSVC support such spelling?

[frederick-vs-ja]

This issue seems not correctly fixed (llvm/llvm-project#61196 (comment)).

[CaseyCarter]

Per Discord discussion, we should probably give up battling the preprocessor and just have the compiler front end implement ugly equivalents of the pertinent attributes.