[ADT] Attempt to fix GCC warning in IntrusiveRefCntPtrTest.

Our copy constructor doesn't explicitly invoke the base class's
constructor, and GCC is (rightly) concerned.

llvm-svn: 291023
diff --git a/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp b/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
index 143a8cc..83dc137 100644
--- a/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
+++ b/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
@@ -15,7 +15,9 @@
 namespace {
 struct SimpleRefCounted : public RefCountedBase<SimpleRefCounted> {
   SimpleRefCounted() { ++NumInstances; }
-  SimpleRefCounted(const SimpleRefCounted &) { ++NumInstances; }
+  SimpleRefCounted(const SimpleRefCounted &) : RefCountedBase() {
+    ++NumInstances;
+  }
   ~SimpleRefCounted() { --NumInstances; }
 
   static int NumInstances;