Tweak diagnostic wording for init list narrowing

The conventional form is '<action> to silence this warning'.

Also call the diagnostic an 'issue' rather than a 'message' because the latter
term is more widely used with reference to message expressions.

llvm-svn: 209052
diff --git a/clang/test/SemaTemplate/constexpr-instantiate.cpp b/clang/test/SemaTemplate/constexpr-instantiate.cpp
index 95d6c23..e8e3e7d 100644
--- a/clang/test/SemaTemplate/constexpr-instantiate.cpp
+++ b/clang/test/SemaTemplate/constexpr-instantiate.cpp
@@ -154,7 +154,7 @@
       constexpr S2() {}
       constexpr operator int() const { return 123456; }
     };
-    int k2 = sizeof(short{S2(S2())}); // expected-error {{cannot be narrowed}} expected-note {{override}}
+    int k2 = sizeof(short{S2(S2())}); // expected-error {{cannot be narrowed}} expected-note {{insert an explicit cast to silence this issue}}
   }
 
   namespace PR12288 {