c++: Fix template deduction for conversion operators with xobj parameters [PR113629]

c++: Fix template deduction for conversion operators with xobj parameters [PR113629]

Unification for conversion operators (DEDUCE_CONV) doesn't perform
transformations like handling forwarding references. This is correct in
general, but not for xobj parameters, which should be handled "normally"
for the purposes of deduction: [temp.deduct.conv] only applies to the
return type of the conversion function.

	PR c++/113629

gcc/cp/ChangeLog:

	* pt.cc (type_unification_real): Only use DEDUCE_CONV for the
	return type of a conversion function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-conv-op.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>