Adding new AST matcher: gnuNullExpr

It matches GNU __null expression.

llvm-svn: 236731
diff --git a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
index 46ddb8d..6f1cf3f 100644
--- a/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
+++ b/clang/unittests/ASTMatchers/ASTMatchersTest.cpp
@@ -2144,6 +2144,10 @@
   EXPECT_TRUE(matches("int* i = nullptr;", nullPtrLiteralExpr()));
 }
 
+TEST(Matcher, GNUNullExpr) {
+  EXPECT_TRUE(matches("int* i = __null;", gnuNullExpr()));
+}
+
 TEST(Matcher, AsmStatement) {
   EXPECT_TRUE(matches("void foo() { __asm(\"mov al, 2\"); }", asmStmt()));
 }