c++: use TARGET_EXPR accessors

While futzing around with PR116416 I noticed that we can use
the _SLOT and _INITIAL macros to make the code more readable.

gcc/c-family/ChangeLog:

	* c-pretty-print.cc (c_pretty_printer::primary_expression): Use
	TARGET_EXPR accessors.
	(c_pretty_printer::expression): Likewise.

gcc/cp/ChangeLog:

	* coroutines.cc (build_co_await): Use TARGET_EXPR accessors.
	(finish_co_yield_expr): Likewise.
	(register_awaits): Likewise.
	(tmp_target_expr_p): Likewise.
	(flatten_await_stmt): Likewise.
	* error.cc (dump_expr): Likewise.
	* semantics.cc (finish_omp_target_clauses): Likewise.
	* tree.cc (bot_manip): Likewise.
	(cp_tree_equal): Likewise.
	* typeck.cc (cxx_mark_addressable): Likewise.
	(cp_build_compound_expr): Likewise.
	(cp_build_modify_expr): Likewise.
	(check_return_expr): Likewise.

Reviewed-by: Jason Merrill <jason@redhat.com>