AST: attempt to rewrite the expression to be more legible

This restores the original spelling of the restriction.  This is more
legible and may be something which VS2017 will handle properly.

Thanks to @drodriguez for suggesting this!

Author: compnerd

include/swift/AST/SimpleRequest.h

@@ -113,9 +113,7 @@ namespace detail {
 
   /// Extract the first, nearest source location from a tuple.
   template<unsigned Index, typename ...Types,
-           typename = typename std::enable_if<sizeof...(Types) - Index
-                                                  ? true
-                                                  : false>::type>
+           typename = typename std::enable_if<(Index < sizeof...(Types))>::type>
   SourceLoc extractNearestSourceLocTuple(const std::tuple<Types...> &value) {
     SourceLoc loc = maybeExtractNearestSourceLoc(std::get<Index>(value));
     if (loc.isValid())