[libcxx] Add a release note for Clang-cl specific features (llvm#80010)

Author: mstorsjo

libcxx/docs/ReleaseNotes/18.rst

@@ -342,3 +342,6 @@ Build System Changes
   them so that they can interoperate with other system-provided libraries that might be using a different unwinding
   library (such as ``libgcc_s``), you should pass ``LIBCXXABI_USE_LLVM_UNWINDER=OFF`` and ``COMPILER_RT_USE_LLVM_UNWINDER=OFF``
   to make sure the system-provided unwinding library is used by the LLVM runtimes.
+
+- In Clang-cl configurations, libc++ can now be built against the static and/or
+  debug MSVC C runtimes