c++ __integer_pack conversion again [PR111357]

c++ __integer_pack conversion again [PR111357]

As Jakub pointed out, the real problem here is that in a partial
substitution we're forgetting the conversion to the type of the non-type
template argument, because maybe_convert_nontype_argument doesn't do
anything with value-dependent arguments.  I'm experimenting with changing
that, but in the meantime we can work around it here.

	PR c++/111357

gcc/cp/ChangeLog:

	* pt.cc (expand_integer_pack): Use IMPLICIT_CONV_EXPR.