rust: use range for inside rust-gcc.cc [PR119341]

rust: use range for inside rust-gcc.cc [PR119341]

There are some places inside rust-gcc.cc which are candidates
to use range for instead of iterators directly. This changes
the locations I saw and makes the code slightly more readable.

gcc/rust/ChangeLog:

	PR rust/119341
	* rust-gcc.cc (function_type): Use range fors.
	(function_type_variadic): Likewise.
	(fill_in_fields): Likewise.
	(statement_list): Likewise.
	(block): Likewise.
	(block_add_statements): Likewise.
	(function_set_parameters): Likewise.
	(write_global_definitions): Likewise.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>