[Sema] Constrain test added in r173873 with expected-error-re

Suggested in post-commit review by Richard Smith.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173880 91177308-0d34-0410-b5e6-96231b3b80d8

test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp

@@ -27,8 +27,7 @@ void f2(constexpr int i) {} // expected-error {{function parameter cannot be con
 struct s2 {
   constexpr int mi1; // expected-error {{non-static data member cannot be constexpr; did you intend to make it const?}}
   static constexpr int mi2; // expected-error {{requires an initializer}}
-  // FIXME: verify that there's no extra suffix in this error. -verify doesn't support anything like that at the moment as far as I know
+  mutable constexpr int mi3 = 3; // expected-error-re {{non-static data member cannot be constexpr$}} expected-error {{'mutable' and 'const' cannot be mixed}}
-  mutable constexpr int mi3; // expected-error {{non-static data member cannot be constexpr}} expected-error {{'mutable' and 'const' cannot be mixed}}
 };
 // typedef
 typedef constexpr int CI; // expected-error {{typedef cannot be constexpr}}