c++: Fix handling of temporaries with consteval ctors and non-trivial dtors [PR104055]

c++: Fix handling of temporaries with consteval ctors and non-trivial dtors [PR104055]

The following testcase is miscompiled.  We see the constructor is immediate,
in build_over_call we trigger:
          if (obj_arg && is_dummy_object (obj_arg))
            {
              call = build_cplus_new (DECL_CONTEXT (fndecl), call, complain);
              obj_arg = NULL_TREE;
            }
which makes call a TARGET_EXPR with the dtor in TARGET_EXPR_CLEANUP,
but then call cxx_constant_value on it.  In cxx_eval_outermost_constant_expr
it triggers the:
      else if (TREE_CODE (t) != CONSTRUCTOR)
        {
          r = get_target_expr_sfinae (r, tf_warning_or_error | tf_no_cleanup);
          TREE_CONSTANT (r) = true;
        }
which wraps the CONSTRUCTOR r into a new TARGET_EXPR, but one without
dtors (I think we need e.g. the TREE_CONSTANT for the callers),
and finally build_over_call uses that.

The following patch fixes that by using get_target_expr instead
of get_target_expr_sfinae + TREE_CONSTANT (r) = true if t is
a TARGET_EXPR with non-NULL TARGET_EXPR_CLEANUP.

2022-01-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104055
	* constexpr.cc (cxx_eval_outermost_constant_expr): If t is a
	TARGET_EXPR with TARGET_EXPR_CLEANUP, use get_target_expr rather
	than get_target_expr_sfinae with tf_no_cleanup, and don't set
	TREE_CONSTANT.

	* g++.dg/cpp2a/consteval27.C: New test.