[flang][openmp] Fix bug in `OmpClause::Hint` clause which was missing to...

[flang][openmp] Fix bug in `OmpClause::Hint` clause which was missing to generate inside in OMP.cpp.inc file.

Before this patch "Hint" isn't found inside the generated file.
./bin/llvm-tblgen --gen-directive-gen ../llvm-project/llvm/include/llvm/Frontend/OpenMP/OMP.td -I ../llvm-project/llvm/include/ > OMP.cpp.in

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D91909