[clang] [ARM64EC] Add support for parsing __vectorcall (PR #87725)

[Casey Carter via cfe-commits]

CaseyCarter wrote:

> @CaseyCarter It feels like we are missing an arm64ec check here, https://github.com/microsoft/STL/blob/be81252ed1f5e5fc6d77faca0b5dbbbdae8143a2/stl/inc/type_traits#L398 unless this was intentional and just not documented?

I have absolutely no idea what's going on here or why MSVC doesn't reject this usage. I've filed https://github.com/microsoft/STL/issues/4596 to track investigating and (most likely) removal.

https://github.com/llvm/llvm-project/pull/87725