[Arm64EC] Improve alignment mangling in arm64ec thunks. (#90115)

In some cases, MSVC's mangling for arm64ec thunks includes the alignment
of a struct. I added some code to try to match... but it never really
worked right. The issues:

- Alignment is only mangled if it's 16 or more (I guess the default is
supposed to be 8).
- Alignment isn't mangled on return values (since the memory is
allocated by the caller).

The current patch leaves hooks to make alignment mangling work... but
doesn't actually ever mangle alignment: clang never actually encodes a
relevant alignment into the IR. Once we get clang to emit the real
size/alignment of structs, we can start emitting it.