Implement [[msvc::no_unique_address]] (#65675)

This implements the [[msvc::no_unique_address]] attribute.

There is not ABI compatibility in this patch because the attribute is
relatively new and there's still some uncertainty in the MSVC version.

Bug: https://github.com/llvm/llvm-project/issues/49358

Also see https://reviews.llvm.org/D157762.