c++: track whether we expect a TARGET_EXPR to be elided

c++: track whether we expect a TARGET_EXPR to be elided

A discussion at Cauldron made me think that with the formalization of copy
elision in C++17, we should be able to determine before optimization which
TARGET_EXPRs will become temporaries and which are initializers.  This patch
implements that: we set TARGET_EXPR_ELIDING_P if it's used as an
initializer, and later check that we were right.

There's an exception in the cp_gimplify_expr check to allow extra
temporaries of non-addressable type: this is used by
gimplify_init_ctor_preeval to materialize subobjects of a CONSTRUCTOR on the
rhs of a MODIFY_EXPR rather than materializing the whole object.  If the
type isn't addressable, there's no way for a program to tell the difference,
so this is a valid optimization.

I considered changing replace_placeholders_for_class_temp_r to check
TARGET_EXPR_ELIDING_P instead of potential_prvalue_result_of, but decided
that would be wrong: if we have an eliding TARGET_EXPR inside a non-eliding
one, we would miss replacing its placeholders.

gcc/cp/ChangeLog:

	* cp-tree.h (TARGET_EXPR_ELIDING_P): New.
	(unsafe_copy_elision_p, set_target_expr_eliding)
	(cp_build_init_expr): Declare.
	* call.cc (unsafe_copy_elision_p): No longer static.
	(build_over_call, build_special_member_call)
	(build_new_method_call): Use cp_build_init_expr.
	* coroutines.cc (expand_one_await_expression)
	(build_actor_fn, flatten_await_stmt, handle_nested_conditionals)
	(await_statement_walker, morph_fn_to_coro): Use cp_build_init_expr.
	* cp-gimplify.cc (cp_gimplify_init_expr)
	(cp_gimplify_expr): Check TARGET_EXPR_ELIDING_P.
	(cp_fold_r): Propagate it.
	(cp_fold): Use cp_build_init_expr.
	* decl.cc (check_initializer): Use cp_build_init_expr.
	* except.cc (build_throw): Use cp_build_init_expr.
	* init.cc (get_nsdmi): Call set_target_expr_eliding.
	(perform_member_init, expand_default_init, expand_aggr_init_1)
	(build_new_1, build_vec_init): Use cp_build_init_expr.
	* method.cc (do_build_copy_constructor): Use cp_build_init_expr.
	* semantics.cc (simplify_aggr_init_expr, finalize_nrv_r)
	(finish_omp_reduction_clause): Use cp_build_init_expr.
	* tree.cc (build_target_expr): Call set_target_expr_eliding.
	(bot_manip): Copy TARGET_EXPR_ELIDING_P.
	* typeck.cc (cp_build_modify_expr): Call set_target_expr_eliding.
	(check_return_expr): Use cp_build_modify_expr.
	* typeck2.cc (split_nonconstant_init_1)
	(split_nonconstant_init): Use cp_build_init_expr.
	(massage_init_elt): Call set_target_expr_eliding.
	(process_init_constructor_record): Clear TARGET_EXPR_ELIDING_P on
	unsafe copy elision.
	(set_target_expr_eliding, cp_build_init_expr): New.