Fix const checking of enum discriminants

Fix const checking of enum discriminants

gcc/rust/ChangeLog:

	* checks/errors/rust-const-checker.cc
	(ConstChecker::visit): Visit the enum items of enums.
	* resolve/rust-ast-resolve-item.cc
	(ResolveItem::visit): Resolve enum discriminants during nr1.0.

gcc/testsuite/ChangeLog:

	* rust/compile/enum_discriminant2.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>