c++: wrong-code with consteval constructor [PR117501]

We've had a wrong-code problem since r14-4140, due to which we
forget to initialize a variable.

In consteval39.C, we evaluate

    struct QQQ q;
  <<cleanup_point <<< Unknown tree: expr_stmt
    QQQ::QQQ (&q, TARGET_EXPR <D.2687, <<< Unknown tree: aggr_init_expr
      5
      __ct_comp
      D.2687
      (struct basic_string_view *) <<< Unknown tree: void_cst >>>
      (const char *) "" >>>>) >>>>>;

into

    struct QQQ q;
  <<cleanup_point <<< Unknown tree: expr_stmt
    {.data={._M_len=42, ._M_str=0}} >>>>>;

and then the useless expr_stmt is dropped on the floor, so q isn't
initialized.  As pre-r14-4140, we need to handle constructors specially.

With this patch, we generate:

    struct QQQ q;
  <<cleanup_point <<< Unknown tree: expr_stmt
    q = {.data={._M_len=42, ._M_str=0}} >>>>>;

initializing q properly.

	PR c++/117501

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_build_init_expr_for_ctor): New.
	(cp_fold_immediate_r): Call it.
	(cp_fold): Break out code into cp_build_init_expr_for_ctor.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval39.C: New test.
	* g++.dg/cpp2a/consteval40.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>