e9d5842ac7
This is a follow up to https://github.com/llvm/llvm-project/pull/98717, which made lock_guard available under _LIBCPP_HAS_NO_THREADS. We can make unique_lock available under similar circumstances. This patch follows the example in #98717, by: - Removing the preprocessor guards for _LIBCPP_HAS_NO_THREADS in the unique_lock header. - providing a set of custom mutex implementations in a local header. - using custom locks in tests that can be made to work under `no-threads`.