[libc++] Make std::unique_lock available with _LIBCPP_HAS_NO_THREADS (#99562)

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`.

0 files changed, 0 insertions, 0 deletions