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

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

> I added test to your branch. However, I noticed that in your branch the `flat_multiset` is not implemented. Could you please rebase main, updated the source code for `flat_multiset` and probably add the test that is similar to the test that I added for `flat_set`?

Let me add the test. It seems we don't need to change lambdas in `flat_multiset`.

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