C++17 copy elision improvements.

C++17 copy elision improvements.

	* call.c (build_temp, convert_like_real): Don't re-copy
	TARGET_EXPR.  Handle packed fields.
	(build_x_va_arg): Wrap it in a TARGET_EXPR.
	(build_over_call): Add sanity check.
	* cvt.c (early_elide_copy): New.
	(ocp_convert): Use it.
	* except.c (build_throw): Use it.
	* init.c (get_nsdmi): Put back the TARGET_EXPR.
	(expand_default_init): Call early_elide_copy.
	* typeck.c (cp_build_modify_expr): Call early_elide_copy.

From-SVN: r240845