c++: ICE with temporary of class type in array DMI [PR109966]

c++: ICE with temporary of class type in array DMI [PR109966]

This ICE started with the fairly complicated r13-765.  We crash in
gimplify_var_or_parm_decl because a stray VAR_DECL leaked there.
The problem is ultimately that potential_prvalue_result_of wasn't
correctly handling arrays and replace_placeholders_for_class_temp_r
replaced a PLACEHOLDER_EXPR in a TARGET_EXPR which is used in the
context of copy elision.  If I have

  M m[2] = { M{""}, M{""} };

then we don't invoke the M(const M&) copy-ctor.

One part of the fix is to use TARGET_EXPR_ELIDING_P rather than
potential_prvalue_result_of.  That unfortunately doesn't handle the
case like

  struct N { N(M); };
  N arr[2] = { M{""}, M{""} };

because TARGET_EXPRs that initialize a function argument are not
marked TARGET_EXPR_ELIDING_P even though gimplify_arg drops such
TARGET_EXPRs on the floor.  We can use a pset to avoid replacing
placeholders in them.

I made an attempt to use set_target_expr_eliding in
convert_for_arg_passing but that regressed constexpr-diag1.C, and does
not seem like a prudent change in stage 4 anyway.

	PR c++/109966

gcc/cp/ChangeLog:

	* typeck2.cc (potential_prvalue_result_of): Remove.
	(replace_placeholders_for_class_temp_r): Check TARGET_EXPR_ELIDING_P.
	Use a pset.  Don't replace_placeholders in TARGET_EXPRs that initialize
	a function argument.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/nsdmi-aggr20.C: New test.
	* g++.dg/cpp1y/nsdmi-aggr21.C: New test.