[ItaniumMangle] Fix mangling of GNU __null in an expression to match GCC

clang/lib/AST/ItaniumMangle.cpp

case Expr::StringLiteralClass: {
     Out << 'L';
     assert(isa<ConstantArrayType>(E->getType()));
     mangleType(E->getType());
     Out << 'E';
     break;
   }
 
   case Expr::GNUNullExprClass:
-    // FIXME: should this really be mangled the same as nullptr?
-    // fallthrough
+    // Mangle as if an integer literal 0.
+    Out << 'L';
+    mangleType(E->getType());
+    Out << "0E";
+    break;
 
   case Expr::CXXNullPtrLiteralExprClass: {
     Out << "LDnE";
     break;
   }
 
   case Expr::PackExpansionExprClass:
     Out << "sp";

clang/test/CodeGenCXX/mangle-exprs.cpp

 }
 
 namespace designated_init {
   struct A { struct B { int b[5][5]; } a; };
   // CHECK-LABEL: define {{.*}} @_ZN15designated_init1fINS_1AEEEvDTtlT_di1adi1bdxLi3EdXLi1ELi4ELi9EEE(
   template<typename T> void f(decltype(T{.a.b[3][1 ... 4] = 9}) x) {}
   void use_f(A a) { f<A>(a); }
 }
+
+namespace null {
+  template <decltype(nullptr) P>
+  void cpp_nullptr(typename enable_if<P == nullptr>::type* = 0) {
+  }
+
+  template <void *P>
+  void gnu_null(typename enable_if<P == __null>::type* = 0) {
+  }
+
+  // CHECK-LABEL: define {{.*}} @_ZN4null11cpp_nullptrILDn0EEEvPN9enable_ifIXeqT_LDnEEvE4typeE
+  template void cpp_nullptr<nullptr>(void *);
+
+  // CHECK-LABEL: define {{.*}} @_ZN4null8gnu_nullILPv0EEEvPN9enable_ifIXeqT_Ll0EEvE4typeE
+  template void gnu_null<nullptr>(void *);
+}