[clang] [llvm] [OpenMP] OpenMP 5.1 "assume" directive parsing support (PR #92731)

[Johannes Doerfert via cfe-commits]

jdoerfert wrote:

This broke the Flang build. You need to add some Flang code for all changes in omp.td. Look for a similar clause in the flang subproject to see what to do, and verify by building Flang. Maybe revert for now until fixed.

https://github.com/llvm/llvm-project/pull/92731