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

[Max Winkler via cfe-commits]

MaxEW707 wrote:

LGTM.

>From memory, its been a while since I've had to ship on ARM64EC, all instances of `__vectorcall` are supposed to be rejected by msvc. That also aligns with the `__vectorcall` docs on MSDN.
I am not sure what clang's policy is with potentially allowing code that "should" be rejected by msvc but since this errors when the function is actually generated this should be fine to support even if msvc gets more strict in the future.

@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 intention and just not documented?

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