c++: ICE with NSDMIs and fn arguments [PR116015]

c++: ICE with NSDMIs and fn arguments [PR116015]

The problem in this PR is that we ended up with

  {.rows=(&<PLACEHOLDER_EXPR struct Widget>)->n,
   .outer_stride=(&<PLACEHOLDER_EXPR struct MatrixLayout>)->rows}

that is, two PLACEHOLDER_EXPRs for different types on the same level
in one { }.  That should not happen; we may, for instance, neglect to
replace a PLACEHOLDER_EXPR due to CONSTRUCTOR_PLACEHOLDER_BOUNDARY on
the constructor.

The same problem happened in PR100252, which I fixed by introducing
replace_placeholders_for_class_temp_r.  That didn't work here, though,
because r_p_for_c_t_r only works for non-eliding TARGET_EXPRs: replacing
a PLACEHOLDER_EXPR with a temporary that is going to be elided will
result in a crash in gimplify_var_or_parm_decl when it encounters such
a loose decl.

But leaving the PLACEHOLDER_EXPRs in is also bad because then we end
up with this PR.

TARGET_EXPRs for function arguments are elided in gimplify_arg.  The
argument will get a real temporary only in get_formal_tmp_var.  One
idea was to use the temporary that is going to be elided anyway, and
then replace_decl it with the real object once we get it.  But that
didn't work out: one problem is that we elide the TARGET_EXPR for an
argument before we create the real temporary for the argument, and
when we get it, the context that this was a TARGET_EXPR for an argument
has been lost.  We're also in the middle end territory now, even though
this is a C++-specific problem.

A solution is to simply stop eliding TARGET_EXPRs whose initializer is
a CONSTRUCTOR.  Such copies can't be (at the moment) elided anyway.  But
not eliding all TARGET_EXPRs would be a pessimization.

	PR c++/116015

gcc/cp/ChangeLog:

	* call.cc (convert_for_arg_passing): Don't set_target_expr_eliding
	when the TARGET_EXPR initializer is a CONSTRUCTOR.

gcc/ChangeLog:

	* gimplify.cc (gimplify_arg): Do not strip a TARGET_EXPR whose
	initializer is a CONSTRUCTOR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/nsdmi-aggr23.C: New test.