[libcxx-commits] [libcxx] [libc++] Make sure `flat_set::key_compare` handle `boolean-testable` correctly (PR #132622)

[Hewill Kang via libcxx-commits]

hewillk wrote:

`flat_multiset` has the same issue, so this PR should also deal with `flat_multiset`.
But let me familiarize myself with the contribution steps of llvm first...

https://github.com/llvm/llvm-project/pull/132622