Emit error if function is overloaded.

Test: check error is reported correclty, hidl_test
Bug: 31758541

Change-Id: If82cb3fbd8c068c6e25cbc131a8102e24cf83988
diff --git a/Interface.h b/Interface.h
index 1c793e7..7f7edc7 100644
--- a/Interface.h
+++ b/Interface.h
@@ -27,7 +27,7 @@
 struct Interface : public Scope {
     Interface(const char *localName, Interface *super);
 
-    void addMethod(Method *method);
+    bool addMethod(Method *method);
 
     bool isInterface() const override;
     bool isBinder() const override;
@@ -35,6 +35,7 @@
     const Interface *superType() const;
 
     const std::vector<Method *> &methods() const;
+    Method *lookupMethod(std::string name) const;
 
     std::string getBaseName() const;