c++: Avoid considering some conversion ops [PR97600]

c++: Avoid considering some conversion ops [PR97600]

Patrick's earlier patch to check convertibility before constraints for
conversion ops wasn't suitable because checking convertibility can also lead
to unwanted instantiations, but it occurs to me that there's a smaller check
we can do to avoid doing normal consideration of the conversion ops in this
case: since we're in the middle of a user-defined conversion, we can exclude
from consideration any conversion ops that return a type that would need an
additional user-defined conversion to reach the desired type: namely, a type
that differs in class-ness from the desired type.

[temp.inst]/9 allows optimizations like this: "If the function selected by
overload resolution can be determined without instantiating a class template
definition, it is unspecified whether that instantiation actually takes
place."

gcc/cp/ChangeLog:

	PR libstdc++/97600
	* call.c (build_user_type_conversion_1): Avoid considering
	conversion functions that return a clearly unsuitable type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-conv3.C: New test.