libstdc++: Fix constraint recursion in basic_const_iterator operator- [PR115046]

libstdc++: Fix constraint recursion in basic_const_iterator operator- [PR115046]

It was proposed in PR112490 to also adjust basic_const_iterator's friend
operator-(sent, iter) overload alongside the r15-7757-g4342c50ca84ae5
adjustments to its comparison operators, but we lacked a concrete
testcase demonstrating fixable constraint recursion there.  It turns out
Hewill Kang's PR115046 is such a testcase!  So this patch makes the same
adjustments to that overload as well, fixing PR115046.  The LWG 4218 P/R
will need to get adjusted too.

	PR libstdc++/115046
	PR libstdc++/112490

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (basic_const_iterator::operator-):
	Replace non-dependent basic_const_iterator function parameter with
	a dependent one of type basic_const_iterator<_It2> where _It2
	matches _It.
	* testsuite/std/ranges/adaptors/as_const/1.cc (test04): New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>