Recommit "Implement [[msvc::no_unique_address]] (#65675)" (#67199)

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.

The recommit changes the attribute definitions so that instead of making
two separate attributes for no_unique_address
and msvc::no_unique_address, it modifies the attributes tablegen emitter
to allow spellings to be target-specific.

This reverts commit 71f9e769.