Move Symbol logic into SymbolGraph

Up to now, the `SymbolGraphASTWalker` was only concerned with one module. This
change prepares for emitting multiple symbol graph files, for each module that
the module of interest extended. There is only one walker, so extract the
symbol logic into `SymbolGraph`, where it can be reused.

rdar://58941718

lib/SymbolGraphGen/SymbolGraphGen.cpp

@@ -50,7 +50,7 @@ symbolgraphgen::emitSymbolGraphForModule(ModuleDecl *M,
   }
 
   llvm::json::OStream J(OS, Options.PrettyPrint ? 2 : 0);
-  Walker.Graph.serialize(Walker, J);
+  Walker.Graph.serialize(J);
 
   return EXIT_SUCCESS;
 }