Test-harness attribute macros are stripped before HIR, blocking duplicate_macro_attributes lint

While porting rustc warn-by-default lints, duplicate_macro_attributes cannot be implemented because the attributes it targets are gone by the time the HIR is built.

rustc scopes duplicate_macro_attributes to the built-in test-harness attribute macros: test, bench, test_case, cfg_eval. A lint needs to scan an item's outer attributes and warn when one of those appears twice.

In gccrs these attributes are stripped before HIR. A function lowered from:

#[test]
#[test]
pub fn foo() {}

has an empty get_outer_attrs() at the lint stage, whereas #[inline] / #[cold] survive. So the lint has nothing to inspect.

If the test-harness attributes were preserved on the HIR (or surfaced some other way), the lint would be straightforward.

Trace the gccrs lowering path where test-harness attributes are stripped before HIR, then inspect the HIR item's get_outer_attrs() behavior at the lint stage. The work is done when attributes such as test, bench, test_case, and cfg_eval remain available, or are surfaced another way, so duplicate occurrences can be inspected by the lint.