Add another Clang TableGen-backend (-gen-clang-diags-options) for emitting

declarations for controlling groups of warnings. Currently this transforms:

  def UnusedMacrosDiags : Option<"unused-macros", [pp_macro_not_used]>;
  
into:

  static const diag::kind UnusedMacrosDiags[] = { diag::pp_macro_not_used };

llvm-svn: 67239