[libc++] Implement C++20's P0476R2: std::bit_cast

Thanks to Arthur O'Dwyer for fixing up some of the tests.

Differential Revision: https://reviews.llvm.org/D75960

NOKEYCHECK=True
GitOrigin-RevId: b1fb3d75c953fa2e02ebddb6ebbf100f99786f0c
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index 8631236..bd999f0 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -338,6 +338,7 @@
 * ``upper_bound``
 * ``lock_guard``'s constructors
 * ``as_const``
+* ``bit_cast``
 * ``forward``
 * ``move``
 * ``move_if_noexcept``