fold-const: Fix implicit cast to nullptr_t typedef [PR123790]

fold-const: Fix implicit cast to nullptr_t typedef [PR123790]

The following testcase is incorrectly rejected, because since r15-6744
it adds build_nop to preserve the exact typedef type and
cxx_eval_constant_expression can't fold NOP_EXPR from integer_zerop of one
NULLPTR_TYPE to another NULLPTR_TYPE, while cxx_eval_constant_expression
relies on fold to do such folding.

I see 3 options to fix that, one is deal with this in the r15-6744 spot
and special case NULLPTR_TYPE there and build_zero_cst instead of build_nop
(with similar condition like in the patch below), another possibility is
special case this in cxx_eval_constant_expression, and
another one (done in this patch) is to handle this in fold-const.cc -
fold_convert_loc and also in fold_convert_const.

2026-01-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123790
	* fold-const.cc (fold_convert_const): Handle conversion of
	integer_zerop to NULLPTR_TYPE.
	(fold_convert_loc): Likewise.

	* g++.dg/cpp0x/nullptr47.C: New test.