[libcxx-commits] [libcxx] Revert "[libc++] Allow the use of extensions in the implementation (#79532)" (PR #85792)

[Sam McCall via libcxx-commits]

sam-mccall wrote:

#79532 has details on the regression.

It seems there are other ways to allow extensions broadly without using push/pop everywhere. But there hasn't been any movement there so far, and this is a large enough compile performance regression that we can't ship libc++.

Contra https://github.com/llvm/llvm-project/pull/79532#issuecomment-1997572838, a clean revert seems to pass tests, so I don't think this is heavily used yet.
So I'd like to revert this until it's clear what the best way to enable this is.

https://github.com/llvm/llvm-project/pull/85792