Implement LWG 2904.

Summary:
- Removed the move-constructibe requirement from copy-assignable.
- Updated `__assign_alt` such that we direct initialize if
  `_Tp` can be `nothrow`-constructible from `_Arg`, or `_Tp`'s
  move construction can throw. Otherwise, construct a temporary and move it.
- Updated the tests to remove the pre-LWG2904 path.

Depends on D32671.

Reviewers: EricWF, CaseyCarter

Reviewed By: EricWF

Differential Revision: https://reviews.llvm.org/D33965

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304891 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/variant/variant.variant/variant.assign/move.pass.cpp

@@ -183,13 +183,7 @@ void test_move_assignment_sfinae() {
   }
   {
     using V = std::variant<int, CopyOnly>;
-#ifdef _LIBCPP_VERSION // LWG2904
-    // variant only provides move assignment when both the move constructor
-    // and move assignment operator are well formed.
-    static_assert(!std::is_move_assignable<V>::value, "");
-#else // _LIBCPP_VERSION // LWG2904
     static_assert(std::is_move_assignable<V>::value, "");
-#endif // _LIBCPP_VERSION // LWG2904
   }
   {
     using V = std::variant<int, NoCopy>;
@@ -232,12 +226,10 @@ void test_move_assignment_sfinae() {
     using V = std::variant<int, TrivialCopyNontrivialMove>;
     static_assert(!std::is_trivially_move_assignable<V>::value, "");
   }
-#ifndef _LIBCPP_VERSION // LWG2904
   {
     using V = std::variant<int, CopyOnly>;
     static_assert(std::is_trivially_move_assignable<V>::value, "");
   }
-#endif // _LIBCPP_VERSION // LWG2904
 }
 
 void test_move_assignment_empty_empty() {