gccrs: add non camel case types lint

[lucasly-ba]

gcc/rust/ChangeLog:

* checks/lints/unused/rust-unused-checker.cc (is_camel_case): Add warning for struct, enum, and trait. (UnusedChecker::visit): New.
* checks/lints/unused/rust-unused-checker.h: New.

gcc/testsuite/ChangeLog:

* rust/compile/non-camel-case-types_0.rs: New test.

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[dkm]

Hello :)
This is not correct:

* checks/lints/unused/rust-unused-checker.h:
	New.

As it can be read as "new file rust-unused-checker.h". A more correct changelog would be:

	* checks/lints/unused/rust-unused-checker.h
	(UnusedChecker::visit(HIR::Trait),UnusedChecker::visit(HIR::StructStruct))
	(UnusedChecker::visit(HIR::TupleStruct),UnusedChecker::visit(HIR::Enum)):
	New.

[P-E-P]

This is a system header, which should not be used. Use rust-system.h instead.

[CohenArthur]

The implementation is great - just one nitpick about code location but otherwise we should merge this :)

[CohenArthur]

I don't know if having this as part of the UnusedChecker is great. I would rather see it as its own pass, or maybe as part of smaller, less-essential lints