Only use explicit copy-ctor in to_narrow with C++17

Summary: [Folly] Only use `explicit` copy-ctor in `to_narrow` with C++17 and higher, since C++17 introduces mandatory copy elision and the technique here relies on mandatory copy elision.

Differential Revision: D19175384

fbshipit-source-id: 9d3234a71b1b5e4f31d14137e42dbf3a73c79b2e

folly/Utility.h

@@ -312,8 +312,13 @@ class to_narrow_convertible {
 
   explicit constexpr to_narrow_convertible(Src const& value) noexcept
       : value_(value) {}
+#if __cplusplus >= 201703L
   explicit to_narrow_convertible(to_narrow_convertible const&) = default;
   explicit to_narrow_convertible(to_narrow_convertible&&) = default;
+#else
+  to_narrow_convertible(to_narrow_convertible const&) = default;
+  to_narrow_convertible(to_narrow_convertible&&) = default;
+#endif
   to_narrow_convertible& operator=(to_narrow_convertible const&) = default;
   to_narrow_convertible& operator=(to_narrow_convertible&&) = default;