[llvm] [CI] Downgrade to clang-20 for libc fullbuild (PR #150246)

[via llvm-commits]

lntue wrote:

Given that the LLVM libc full build pre-commit CI has been down for quite some time, and there is no apparent progress on the github issues, I'm going to approve downgrading to llvm-20 in the meantime to make these useful and reduce the noise.

https://github.com/llvm/llvm-project/pull/150246