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 71f9e7695b87298f9855d8890f0e6a3b89381eb5.
This reverts commit 4a55d426967b9c70f5dea7b3a389e11393a4f4c4.
Reverting because this breaks sphinx documentation, and even with it
fixed the format of the attribute makes the no_unique_address
documentation show up twice.
