[clang] [Clang] Remove workaround for libstdc++4.7 (PR #139693)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman We previously removed other such hacks, do you think a release note is still useful [6ad7e87](https://github.com/llvm/llvm-project/commit/6ad7e87806c0af774cf2f8880694f79259925979) ?

Yeah, I think a release note is still handy, just so users aren't surprised.

https://github.com/llvm/llvm-project/pull/139693