c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement ...

c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]

signed char, char or short int pre/post inc/decrement are represented by
normal {PRE,POST}_{INC,DEC}REMENT_EXPRs in the FE and only gimplification
ensures that the {PLUS,MINUS}_EXPR is done in unsigned version of those
types:
    case PREINCREMENT_EXPR:
    case PREDECREMENT_EXPR:
    case POSTINCREMENT_EXPR:
    case POSTDECREMENT_EXPR:
      {
        tree type = TREE_TYPE (TREE_OPERAND (*expr_p, 0));
        if (INTEGRAL_TYPE_P (type) && c_promoting_integer_type_p (type))
          {
            if (!TYPE_OVERFLOW_WRAPS (type))
              type = unsigned_type_for (type);
            return gimplify_self_mod_expr (expr_p, pre_p, post_p, 1, type);
          }
        break;
      }
This means during constant evaluation we need to do it similarly (either
using unsigned_type_for or using widening to integer_type_node).
The following patch does the latter.

2022-10-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105774
	* constexpr.cc (cxx_eval_increment_expression): For signed types
	that promote to int, evaluate PLUS_EXPR or MINUS_EXPR in int type.

	* g++.dg/cpp1y/constexpr-105774.C: New test.