Change crate name retrieval function return types

Change crate name retrieval function return types

Change their return type to a const reference in order to avoid copies
when possible. Also wrap this new return type into an optional.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-borrow-checker.cc (BorrowChecker::go):
	Adapt the code to the new return types.
	* resolve/rust-ast-resolve.cc (NameResolution::go): Likewise.
	* util/rust-hir-map.cc (Mappings::get_crate_name): Change return type
	to const string reference optional.
	(Mappings::get_current_crate_name): Likewise.
	* util/rust-hir-map.h: Update function prototypes.

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