gccrs: Refactor handle_substitutions to take a reference

gccrs: Refactor handle_substitutions to take a reference

This patch changes the recusive substitution code to take a reference
instead of a copy. This is important as the callback field is going to be
made non-copyable in a future patch and this pipeline is for recursive
substitutions so its ok to reuse the same mappings here.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>
gcc/rust/ChangeLog:

	* typecheck/rust-tyty-bounds.cc: refactor to take a reference
	* typecheck/rust-tyty-subst.cc: likewise
	(SubstitutionRef::get_substitution_arguments): likewise
	(SubstitutionRef::infer_substitions): likewise
	* typecheck/rust-tyty-subst.h: likewise
	* typecheck/rust-tyty.cc (ADTType::handle_substitions): likewise
	(TupleType::handle_substitions): likewise
	(FnType::handle_substitions): likewise
	(ClosureType::handle_substitions): likewise
	(ArrayType::handle_substitions): likewise
	(SliceType::handle_substitions): likewise
	(ReferenceType::handle_substitions): likewise
	(PointerType::handle_substitions): likewise
	(ParamType::handle_substitions): likewise
	(ProjectionType::handle_substitions): likewise
	* typecheck/rust-tyty.h: likewise