[clang] [Clang] Add __ugly__ spelling for the msvc attribute scope (PR #113765)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I'm personally sympathetic, and would be ok with something that we know MSVC won't step on (`__clang_msvc__` would be fine to me). As far as the `no_unique_address`, I THINK we already get the double-underscore spelling of that, don't we? I don't think we exclude the MSVC spellings from that.
> 
> AFAICT you're only allowed to __uglify__ attributes with no namespace, `clang`, or `gnu`. At least clang 18 rejects `msvc::__no_unique_address__` with a warning: https://godbolt.org/z/zq89ssnh1

I see that now :/  I managed to mis-remember that, and did the research AFTER posting!

https://github.com/llvm/llvm-project/pull/113765