[frontend] Introduce a new frontend option '-dump-api-path', which outputs a swift interface file for each compiled source file.

This is primarily intended for use with the stdlib.

lib/Frontend/CompilerInvocation.cpp

@@ -111,6 +111,10 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     }
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_dump_api_path)) {
+    Opts.DumpAPIPath = A->getValue();
+  }
+
   Opts.EmitVerboseSIL |= Args.hasArg(OPT_emit_verbose_sil);
   Opts.EmitSortedSIL |= Args.hasArg(OPT_emit_sorted_sil);
 
@@ -650,6 +654,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                           DiagnosticEngine &Diags, bool isImmediate) {
   using namespace options;
 
+  Opts.AttachCommentsToDecls |= Args.hasArg(OPT_dump_api_path);
+
   Opts.UseMalloc |= Args.hasArg(OPT_use_malloc);
 
   Opts.EnableExperimentalPatterns |=