Fix GCC's pedantic return-type warning -- this enum is fully covered.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154399 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Parse/ParseDecl.cpp

@@ -937,6 +937,7 @@ bool Parser::DiagnoseProhibitedCXX11Attribute() {
       << SourceRange(BeginLoc, EndLoc);
     return true;
   }
+  llvm_unreachable("All cases handled above.");
 }
 
 void Parser::DiagnoseProhibitedAttributes(ParsedAttributesWithRange &attrs) {