[clang] [clang] check deduction consistency when partial ordering function templates (PR #100692)

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

Please wait until tomorrow in case @zygoloid @hokein @zyn0217 have further comments but I think this is in good shape, thanks

I would like to see simplifications of the APIs in subsequent PRs (too many bools, out pointers, and duplication)

Also, can you make sure that libc++ tests pass before merging?
This is technically a breaking change.

https://github.com/llvm/llvm-project/pull/100692