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

[via cfe-commits]

h-vetinari wrote:

I've [tested](https://github.com/conda-forge/clangdev-feedstock/pull/333) this PR (as of 2bcc4e5f7043dab1ef673dd20b38009363db51db) in our infrastructure and can confirm that things run fine with VS2019 again. Thanks a lot for reworking this! 🙏 

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