gccrs: add non_contiguous_range_endpoints lint

Warn when an exclusive range pattern and the next arm are one value
apart, such as `20..30` followed by `31..=40`, which silently skips 30.

gcc/rust/ChangeLog:

	* checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
	New.
	* checks/lints/unused/rust-unused-checker.h (UnusedChecker::visit):
	New.

gcc/testsuite/ChangeLog:

	* rust/compile/non-contiguous-range-endpoints_0.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>