When converting some of the old Migrator automation to the new Migrator,

I had set up the driver to invoke a separate frontend invocation with
the "update code" mode. We sort of did this last release, except we
forked to the swift-update binary instead. This is causing problems with
testing in Xcode.

Instead, let's perform a single compile and add the remap file as an
additional output during normal compiles. The driver, seeing
-update-code, will add -emit-remap-file-path $PATH to the -c frontend
invocation.

rdar://problem/31857580

lib/FrontendTool/FrontendTool.cpp

@@ -460,16 +460,17 @@ static bool performCompile(std::unique_ptr<CompilerInstance> &Instance,
 
   ASTContext &Context = Instance->getASTContext();
 
+  if (!Context.hadError() &&
+      Invocation.getMigratorOptions().shouldRunMigrator()) {
+    migrator::updateCodeAndEmitRemap(*Instance, Invocation);
+  }
+
   if (Action == FrontendOptions::REPL) {
     runREPL(*Instance, ProcessCmdLine(Args.begin(), Args.end()),
             Invocation.getParseStdlib());
     return Context.hadError();
   }
 
-  if (Action == FrontendOptions::UpdateCode) {
-    return migrator::updateCodeAndEmitRemap(Invocation);
-  }
-
   SourceFile *PrimarySourceFile = Instance->getPrimarySourceFile();
 
   // We've been told to dump the AST (either after parsing or type-checking,