[ClangImporter] Simply clang-importer creation flags

Currently,  `-direct-clang-cc1-module-build` and `-only-use-extra-clang-opts`
have to be passed together for clang importer creation to succeed.
Missing either will result in error. Simplified the swift-frontend flags
by removing `-only-use-extra-clang-opts` and let
`-direct-clang-cc1-module-build` to do both.

lib/Frontend/CompilerInvocation.cpp

@@ -1655,7 +1655,6 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
     Opts.EnableClangSPI = false;
   }
 
-  Opts.ExtraArgsOnly |= Args.hasArg(OPT_extra_clang_options_only);
   Opts.DirectClangCC1ModuleBuild |= Args.hasArg(OPT_direct_clang_cc1_module_build);
 
   if (const Arg *A = Args.getLastArg(OPT_pch_output_dir)) {