Implement parser/IR support for CFG functions, basic blocks and return instruction.

This is pretty much minimal scaffolding for this step.  Basic block arguments,
instructions, other terminators, a proper IR representation for
blocks/instructions, etc are all coming.

PiperOrigin-RevId: 201826439
diff --git a/include/mlir/IR/Module.h b/include/mlir/IR/Module.h
index 037e288..bf77078 100644
--- a/include/mlir/IR/Module.h
+++ b/include/mlir/IR/Module.h
@@ -34,8 +34,8 @@
   std::vector<Function*> functionList;
 
 
-  void print(raw_ostream &os);
-  void dump();
+  void print(raw_ostream &os) const;
+  void dump() const;
 };
 } // end namespace mlir