[clang] [clang] Remove the experimental prefix from -fexperimental-relative-c++-abi-vtables (PR #142845)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

The changes should come with a release note to let the users know this is no longer experimental.

That said, I think we're still missing something for this to no longer be experimental: documentation. I can't find any mention of this flag in `LanguageExtensions.rst` and I think users would appreciate an explanation of why this feature should be used.

https://github.com/llvm/llvm-project/pull/142845