clang-format: Fix segfault in overloaded operator parsing.

Before, constructs like:
  using A::operator+;

caused a segfault. This fixes llvm.org/PR17050.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189749 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/Format/FormatTest.cpp b/unittests/Format/FormatTest.cpp
index 51577a6..bdbc155 100644
--- a/unittests/Format/FormatTest.cpp
+++ b/unittests/Format/FormatTest.cpp
@@ -3625,6 +3625,8 @@
 
   verifyGoogleFormat("operator void*();");
   verifyGoogleFormat("operator SomeType<SomeType<int>>();");
+
+  verifyFormat("using A::operator+;");
 }
 
 TEST_F(FormatTest, UnderstandsNewAndDelete) {