[ARM64EC] Add support for parsing __vectorcall (#87725)
MSVC doesn't support generating __vectorcall calls in Arm64EC mode, but
it does treat it as a distinct type. The Microsoft STL depends on this
functionality. (Not sure if this is intentional.) Add support for
parsing the same way as MSVC, and add some checks to ensure we don't try
to actually generate code.
The error handling in CodeGen is ugly, but I can't think of a better way
to do it.
