gccrs: Change lookup_canonical_path's return path

gccrs: Change lookup_canonical_path's return path

Change the function's return type to wrap it within an optional.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: Change call site to accomodate new
	return type.
	* backend/rust-compile-base.h: Change parameter to use a reference.
	* backend/rust-compile-extern.h: Likewise.
	* backend/rust-compile-implitem.cc (CompileTraitItem::visit):
	Change call site for new return type.
	* backend/rust-compile-item.cc (CompileItem::visit): Likewise.
	* resolve/rust-ast-resolve-type.cc (ResolveTypeToCanonicalPath::visit):
	Likewise.
	* typecheck/rust-hir-type-check-enumitem.cc (TypeCheckEnumItem::visit):
	Likewise.
	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItem::visit):
	Likewise.
	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit):
	Likewise.
	* typecheck/rust-hir-type-check.cc (TraitItemReference::get_type_from_fn):
	Likewise.
	* util/rust-hir-map.h: Update the function's prototype and change the
	function's return type.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>