Fix tests. We need to make the decoy function opaque since we now allow a function which just returns nullptr.

Author: rniwa

clang/test/Analysis/Checkers/WebKit/call-args-checked-ptr.cpp

@@ -117,7 +117,7 @@ namespace null_ptr {
 
 namespace ref_counted_lookalike {
   struct Decoy {
-    CheckedObj* get() { return nullptr; }
+    CheckedObj* get();
   };
 
   void foo() {

clang/test/Analysis/Checkers/WebKit/call-args.cpp

@@ -117,7 +117,7 @@ namespace null_ptr {
 
 namespace ref_counted_lookalike {
   struct Decoy {
-    RefCountable* get() { return nullptr; }
+    RefCountable* get();
   };
 
   void foo() {