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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman I'm not _super_ worried about these specific cases, but I'd like to know how to proceed with `msvc` attributes we'd like to use in libc++ generally. 

I think there's three options.

1) Don't use `msvc` attributes in libc++ as they're not fit for use in system headers.
2) Use the `msvc` attributes in libc++ and do not support any uses where `msvc` attributes (or the vendor prefix) are in conflict with user macros.
3) Expose the `msvc` attributes under a new vendor prefix (in addition to the `msvc` namespace) and use those from libc++. This includes spelling it `__msvc__` or `_Msvc` (lol).

As a non-libc++ person, it's easy for me to support (1). :-D I really think Microsoft needs to address this issue on their end. Also as a non-libc++ person, I could live with (2). I'm not keen on (3) because this would mean that Clang has multiple vendor prefixes that mean "Clang" and makes it a bit confusing as to whether the attributes under the new vendor prefix are ours or Microsoft's when conflicts arise. e.g., we start having the risk of Clang-specific behaviors under one vendor spelling and Microsoft-specific behaviors under the other.

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