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

[tianshilei1992]

Summary

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.

[tianshilei1992]

rebase

[jdoerfert]

LG

clang/include/clang/Driver/Options.td

defm objc_avoid_heapify_local_blocks : BoolFOption<"objc-avoid-heapify-local-blocks",
   PosFlag<SetTrue, [], "Try">,
   NegFlag<SetFalse, [], "Don't try">,
   BothFlags<[CC1Option, NoDriverOption], " to avoid heapifying local blocks">>;
 
 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],
           "Enable all Clang extensions for OpenMP directives and clauses">,
   NegFlag<SetFalse, [CC1Option, NoArgumentUnused],
           "Disable all Clang extensions for OpenMP directives and clauses">>;
 def fopenmp_EQ : Joined<["-"], "fopenmp=">, Group<f_Group>;
 def fopenmp_use_tls : Flag<["-"], "fopenmp-use-tls">, Group<f_Group>,