Add -import-underlying-module option.

This option implicitly imports the Clang module with the same name as the
module being built into every source file in the module being built.
This will be used for mixed-source framework targets to give Swift code the
same implicit visibility for Objective-C decls in the same module that it
already has for other Swift decls.

<rdar://problem/16701230>

Swift SVN r17053

lib/Frontend/CompilerInvocation.cpp

@@ -493,6 +493,7 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
   Opts.EnableSourceImport = Args.hasArg(OPT_enable_source_import);
   Opts.SILSerializeAll = Args.hasArg(OPT_sil_serialize_all);
   Opts.LLVMArgs = Args.getAllArgValues(OPT_Xllvm);
+  Opts.ImportUnderlyingModule = Args.hasArg(OPT_import_underlying_module);
 
   return false;
 }