[Request-evaluator] Add GraphViz output option -output-request-graphviz.

Introduce a command-line option to visualize the complete set of output
request dependencies evaluated by a particular compile action. This is
exposing existing visualization facilities to the (-frontend) command line.

lib/Serialization/SerializedModuleLoader.cpp

@@ -702,4 +702,4 @@ SerializedASTFile::getDiscriminatorForPrivateValue(const ValueDecl *D) const {
   Identifier discriminator = File.getDiscriminatorForPrivateValue(D);
   assert(!discriminator.empty() && "no discriminator found for value");
   return discriminator;
-}
+}