folly/FixedString: leverage c++14 to support GCC 8

Summary:
gcc8 fails compilation with:
    error: taking address of temporary array
      return {+A{Cs..., Char(0)}, sizeof...(Cs)};
So leverage c++14 to base manipulations on a local array.

Reviewed By: yfeldblum, ericniebler, Orvid

Differential Revision: D9310662

fbshipit-source-id: 0ba375ac0a4cca913e437290bc277c0c6bb8fdc8

folly/FixedString.h

@@ -3037,9 +3037,14 @@ constexpr const std::size_t& npos = detail::fixedstring::FixedStringBase::npos;
  */
 template <class Char, Char... Cs>
 constexpr BasicFixedString<Char, sizeof...(Cs)> operator"" _fs() noexcept {
+#if __cplusplus >= 201402L
+  const Char a[] = {Cs..., Char(0)};
+  return {+a, sizeof...(Cs)};
+#else
   using A = const Char[sizeof...(Cs) + 1u];
   // The `+` in `+A{etc}` forces the array type to decay to a pointer
   return {+A{Cs..., Char(0)}, sizeof...(Cs)};
+#endif
 }
 
 #pragma GCC diagnostic pop