[clang] [clang] Fix sorting module headers (PR #73146)

[David Blaikie via cfe-commits]

dwblaikie wrote:

> OK. I modified this PR in order to only make the changes that fix #73145 . Sorting will remain broken as it has been since 2021. I will provide another PR after this one gets merged.

Thanks, really appreciate your patience/understanding here - I know it's a bit of a fuss to split things up, repost, etc. 

https://github.com/llvm/llvm-project/pull/73146