libstdc++: Fix test that fails in C++20 mode

libstdc++: Fix test that fails in C++20 mode

This test was written to verify that the LWG 3265 changes work. But
those changes were superseded by LWG 3435, and the test is now incorrect
according to the current draft. The assignment operator is now
constrained to also require convertibility, which makes the test fail.

Change the Iter type to be convertible from int*, but make it throw an
exception if that conversion is used. Change the test from compile-only
to run, so we verify that the exception isn't thrown.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/move_iterator/dr3265.cc: Fix test to
	account for LWG 3435 resolution.