FindOpenMP: Add support for openmp 5.1 (llvm 17) and 5.2

llvm 17 now defaults to openmp 5.1; see LLVM commit `0c6f2f629cc0`.
It makes FindOpenMP set OPENMP_FOUND and OpenMP_C_SPEC_DATE, but not
OpenMP_C_VERSION, because it doesn't know the version mapping.
Add the spec date to the version mapping for 5.1 and 5.2 to make
things work again.