[libcxx-commits] [libcxx] [libc++][test] Change forbidden `extents<char>` to `extents<signed char>` (PR #73535)

[Louis Dionne via libcxx-commits]

ldionne wrote:

@crtrott Can you follow-up on adding a `static_assert` for the mandate? We should do this sooner rather than later since it'll prevent people from starting to rely on this.

https://github.com/llvm/llvm-project/pull/73535