[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

Author: shiltian

clang/include/clang/Driver/Options.td

@@ -2395,7 +2395,8 @@ def fomit_frame_pointer : Flag<["-"], "fomit-frame-pointer">, Group<f_Group>;
 def fopenmp : Flag<["-"], "fopenmp">, Group<f_Group>, Flags<[CC1Option, NoArgumentUnused, FlangOption, FC1Option]>,
   HelpText<"Parse OpenMP pragmas and generate parallel code.">;
 def fno_openmp : Flag<["-"], "fno-openmp">, Group<f_Group>, Flags<[NoArgumentUnused]>;
-def fopenmp_version_EQ : Joined<["-"], "fopenmp-version=">, Group<f_Group>, Flags<[CC1Option, NoArgumentUnused]>;
+def fopenmp_version_EQ : Joined<["-"], "fopenmp-version=">, Group<f_Group>, Flags<[CC1Option, NoArgumentUnused]>,
+  HelpText<"Set OpenMP version (e.g. 45 for OpenMP 4.5, 50 for OpenMP 5.0). Default value is 50.">;
 defm openmp_extensions: BoolFOption<"openmp-extensions",
   LangOpts<"OpenMPExtensions">, DefaultTrue,
   PosFlag<SetTrue, [CC1Option, NoArgumentUnused],