c: Fix up RAW_DATA_CST handling in check_constexpr_init [PR120057]

c: Fix up RAW_DATA_CST handling in check_constexpr_init [PR120057]

The pr120057-1.c testcase is incorrectly rejected since
 r15-4377 (and for a while it also ICEd after the error), i.e.
the optimization of large C initializers using RAW_DATA_CST.
Similarly, the embed-18.c testcase is incorrectly rejected since
the embed support has been introduced and RAW_DATA_CST used for that.

The callers of check_constexpr_init (store_init_value and
output_init_element) compute int_const_expr as
      int_const_expr = (TREE_CODE (init) == INTEGER_CST
                        && !TREE_OVERFLOW (init)
                        && INTEGRAL_TYPE_P (TREE_TYPE (init)));
but that is only passed through down to check_constexpr_init.
I think tweaking those 2 callers to also allow RAW_DATA_CST for
int_const_expr when check_constexpr_init needs top special case it
no matter what would be larger, so the patch just changes
check_constexpr_init to deal with RAW_DATA_CST in the initializers.
For TYPE_UNSIGNED char precision integral types RAW_DATA_CST is
always valid, for !TYPE_UNSIGNED we need to check for 128-255 values
being turned into negative ones.

2025-05-02  Jakub Jelinek  <jakub@redhat.com>

	PR c/120057
	* c-typeck.cc (check_constexpr_init): Handle RAW_DATA_CST.

	* gcc.dg/cpp/embed-18.c: New test.
	* gcc.dg/pr120057-1.c: New test.
	* gcc.dg/pr120057-2.c: New test.