Implement P0739R0: 'Some improvements to class template argument deduction integration into the standard library' This is an API change (not ABI change) due to a late change in the c++17 standard

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@309296 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/variant/variant.variant/variant.ctor/copy.pass.cpp

@@ -261,4 +261,9 @@ int main() {
   test_copy_ctor_valueless_by_exception();
   test_copy_ctor_sfinae();
   test_constexpr_copy_ctor_extension();
+{ // This is the motivating example from P0739R0
+  std::variant<int, double> v1(3);
+  std::variant v2 = v1;
+  (void) v2;
+}
 }