ModuleInterface: teach -compile-module-from-interface to emit forwarding module

-compile-module-from-interface action now takes arguments of -candidate-module-file.
If one of the candidate module files is up-to-date, the action emits a forwarding
module pointing to the candidate module instead of building a binary module.

lib/Frontend/CompilerInvocation.cpp

@@ -900,6 +900,10 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts,
   }
   if (const Arg *A = Args.getLastArg(OPT_explict_swift_module_map))
     Opts.ExplicitSwiftModuleMap = A->getValue();
+  for (auto A: Args.filtered(OPT_candidate_module_file)) {
+    Opts.CandidateCompiledModules.push_back(resolveSearchPath(A->getValue()));
+  }
+
   // Opts.RuntimeIncludePath is set by calls to
   // setRuntimeIncludePath() or setMainExecutablePath().
   // Opts.RuntimeImportPath is set by calls to