[clang] [flang] [llvm] [CLANG][OpenMP] Add support for OpenMP6.0 transparent clause (PR #166810)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

Undo all changes in flang/*, ClauseT.h, and restore the "isValueOptional" in OMP.td.  That should make it ok.

https://github.com/llvm/llvm-project/pull/166810