[clang] Implement [[msvc::no_unique_address]] (PR #65675)

[Amy Huang via cfe-commits]

amykhuang wrote:

> > We should consider whether we want to support `__msvc_no_unique_address__` or similar as an alternative spelling #61196
> 
> I think `[[__msvc__::__no_unique_address__]]` would be better. This is how the clang and gnu attributes are handled too. I'm also fine with handling this in a follow-up patch.

Sounds good, I can do this in a separate patch

https://github.com/llvm/llvm-project/pull/65675