Add a missing SFINAE condition to the variant's converting constructor.

Remarks: This function shall not participate in overload resolution unless
         `is_same_v<decay_t<T>, variant>` is false, unless `decay_t<T>` is
         neither a specialization of `in_place_type_t` nor a specialization of
         `in_place_index_t`, unless `is_constructible_v<Tj, T>` is true, and
         unless the expression `FUN(std::forward<T>(t))` (with `FUN` being the
         above-mentioned set of imaginary functions) is well formed.

Depends on D34111.

Reviewers: EricWF, K-ballo

Reviewed By: EricWF

Subscribers: fhahn

Differential Revision: https://reviews.llvm.org/D34112

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305668 91177308-0d34-0410-b5e6-96231b3b80d8

include/variant

@@ -1116,6 +1116,8 @@ public:
   template <
       class _Arg,
       enable_if_t<!is_same_v<decay_t<_Arg>, variant>, int> = 0,
+      enable_if_t<!__is_inplace_type<decay_t<_Arg>>::value, int> = 0,
+      enable_if_t<!__is_inplace_index<decay_t<_Arg>>::value, int> = 0,
       class _Tp = __variant_detail::__best_match_t<_Arg, _Types...>,
       size_t _Ip =
           __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,