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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

The changes should come with a release note mentioning that we've removed the hack because it's no longer a supported libstdc++ (for the few users still relying on the hack).

Otherwise, LGTM!

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