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

llvm-svn: 154399

clang/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) {