c++: reduce redundant TARGET_EXPR

c++: reduce redundant TARGET_EXPR

An experiment led me to notice that in some cases we were ending up with
TARGET_EXPR initialized by TARGET_EXPR, which isn't useful.

The target_expr_needs_replace change won't make a difference in most cases,
since cp_genericize_init will have already expanded VEC_INIT_EXPR by the
time we consider it, but it is correct.

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r) [TARGET_EXPR]: Collapse
	TARGET_EXPR within TARGET_EXPR.
	* constexpr.cc (cxx_eval_outermost_constant_expr): Avoid
	adding redundant TARGET_EXPR.
	* cp-tree.h (target_expr_needs_replace): VEC_INIT_EXPR doesn't.