[clang] Patch series to reapply #118734 and substantially improve it (PR #120534)

[via cfe-commits]

dyung wrote:

> All of the dependent changes have landed, and I've re-structured this patch series to hopefully be easier to review. I still don't have a good way to land this incrementally, as the tools to work around MSVC limitations are only available when using TableGen-ed string tables, and those aren't available at first.
> 
> I could split out some of the later patches, but those are smaller and probably doesn't help as much. But the patches should be meaningful chunks to review here. There have been quite a few changes, so this does probably warrant a re-review from folks. Sorry for that.
> 
> @dyung -- as always, appreciate help verifying that this isn't going to break MSVC builds for you. The top of this PR should be up to date and ready to test.

Happy to do so! Some updates on our end, I *think* that we have finished updating our internal machines with a newer build of Visual Studio 2019, but I need to confirm it. Our upstream bot has not yet been updated though, but I can do that pretty quickly/easily. I will test your changes on both versions tomorrow and let you know the results.

https://github.com/llvm/llvm-project/pull/120534