clang-tidy/readability-identifier-naming: crash on DependentTemplateSpecializationType

Summary:
Previously, the added test cases crashed because the passed a null Decl
to addUsage().

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D22188

llvm-svn: 274985
diff --git a/clang-tools-extra/test/clang-tidy/readability-identifier-naming.cpp b/clang-tools-extra/test/clang-tidy/readability-identifier-naming.cpp
index 4253248..3e7e0b3 100644
--- a/clang-tools-extra/test/clang-tidy/readability-identifier-naming.cpp
+++ b/clang-tools-extra/test/clang-tidy/readability-identifier-naming.cpp
@@ -344,3 +344,9 @@
 // CHECK-FIXES: {{^}}void MY_TEST_MACRO(function) {}
 }
 }
+
+template <typename t_t> struct a {
+  typename t_t::template b<> c;
+};
+
+template <template <typename> class A> struct b { A<int> c; };