Made the "expected string literal" diagnostic more expressive

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168267 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index d898447..7c5b153 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -735,7 +735,8 @@
     ConsumeToken();
     if (Keyword == Ident_message) {
       if (!isTokenStringLiteral()) {
-        Diag(Tok, diag::err_expected_string_literal);
+        Diag(Tok, diag::err_expected_string_literal)
+          << /*Source='availability attribute'*/2;
         SkipUntil(tok::r_paren);
         return;
       }