[flang] [llvm] [flang][OpenMP] Make OpenMP clause representation language-agnostic (PR #86289)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

@skatrak @kiranchandramohan I've merged it since it's NFC, and Tom has reviewed it, plus I have more work based on it.  Still, feel free to leave post-commit review comments if you want.

https://github.com/llvm/llvm-project/pull/86289