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

[via cfe-commits]

dyung wrote:

> @dyung -- Ok, my latest attempt to work around these MSVC issues is now pushed to this PR. It also contains a commit of specifically debugging hacks to try and extract more information from any failure here. If you could try doing another build with the latest commit ([2ec750d](https://github.com/llvm/llvm-project/pull/120534/commits/2ec750d0105e1d4d52791f85293d16d28f6ccbd8)) in this PR, would be very interested in how it works.

```
fatal error: error in backend: Name for __nvvm_vote_all is: ''!!!
```
I don't see any instances of the `__builtin_ia32_packsswb128` from a quick search of the test log.

Here is a compressed copy of the test log file:    [log_test.2ec750d0105e1d4d52791f85293d16d28f6ccbd8.zip](https://www.dropbox.com/scl/fi/60gizuhvi85lz8jxw44t9/log_test.2ec750d0105e1d4d52791f85293d16d28f6ccbd8.zip?rlkey=esic3xnyg4i11rjzuc9i0cr2a&dl=0)

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