Auto merge of #79068 - DevJPM:ci-llvm-9-fix, r=pietroalbini

Remove Hacks and Fixmes from PR CI's LLVM-9 Container

Now with LLVM 9 being the minimum supported version (thanks to #78848 ), we can
finally remove the hacks in the dockerfile.

This wasn't in the main PR bumping the version as I didn't quite
understand what's going on and needed here.

Relevant issues and PRs:

- Issue #69823
- PR #70989

I hope I actually adressed things correctly here?