[docs] Improve help for OpenMP options, NFC.

 * Add HelpText for -fopenmp so that it appears in clang --help.
 * Hide -fno-openmp-simd, only list the positive option.
 * Hide -fopenmp-relocatable-target and -fopenmp-use-tls from
   clang --help and from ClangCommandLineReference.
 * Improve MetaVarName for -Xopenmp-target=<...>.

Differential Revision: https://reviews.llvm.org/D42841

llvm-svn: 325806