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

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EricWF wrote:

If it is because of the diagnostic overrides, we should be able to go back to simply using the pragma system header.

The diagnostic disabling is only really for our test suite (and people who enable -Wsystem-header, but they asking to see diagnostics). We should just be able to disable the warning entirely in the test suite. IDK Why we have it on when we allow the use of extensions. 

@philnik777 Can you say why you chose to go this route?

https://github.com/llvm/llvm-project/pull/79532