Fix ASTMatchersTests to not create an overloaded-virtual warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166921 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/ASTMatchers/ASTMatchersTest.h b/unittests/ASTMatchers/ASTMatchersTest.h
index 66cc9bd..5e63b6b 100644
--- a/unittests/ASTMatchers/ASTMatchersTest.h
+++ b/unittests/ASTMatchers/ASTMatchersTest.h
@@ -24,10 +24,8 @@
 class BoundNodesCallback {
 public:
   virtual ~BoundNodesCallback() {}
-  virtual bool run(const BoundNodes *BoundNodes) { return false; }
-  virtual bool run(const BoundNodes *BoundNodes, ASTContext *Context) {
-    return run(BoundNodes);
-  }
+  virtual bool run(const BoundNodes *BoundNodes) = 0;
+  virtual bool run(const BoundNodes *BoundNodes, ASTContext *Context) = 0;
 };
 
 // If 'FindResultVerifier' is not NULL, sets *Verified to the result of