[Clang] Add helper text to fopenmp_version_EQ to make it show in help menu

For now if we check `clang --help`, it doesn't show `-fopenmp-version`. This option
should be visible to users. In addition, it is not set to hidden in
`clang/include/clang/Driver/Options.td` as well. The reason it doesn't show is
there is no corresponding helper text. This patch simply adds it.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D115998