False positive deprecation and availability warnings on annotated `using` alias declarations

When a `using` alias declaration is annotated with the same attribute as its aliased type, the compiler incorrectly warns that the aliased type is unavailable/deprecated.

```
// Deprecation
struct [[deprecated]] Foo {};
using Bar [[deprecated]] = Foo; // spurious: 'Foo' is deprecated
```

A natural use case is deprecating a chain of aliases together: if `Foo` and `Bar` were previously unannotated and you later decide to deprecate both, you should be able to annotate them without triggering warnings on the alias definitions themselves.

```
// Availability
using FeatureAlias __attribute__((availability(macosx, introduced=11.0))) = int;
using FeatureAlias2 __attribute__((availability(macosx, introduced=11.0))) = FeatureAlias; // spurious: 'FeatureAlias' is only available on macOS 11.0 or newer
```

In both cases the annotation on the `using` declaration should suppress the diagnostic.

Start by compiling the reported deprecation and availability snippets and confirm the diagnostics on the annotated using aliases. Trace the compiler's handling of attributes on using declarations and compare both cases. Done means the alias annotations suppress the warnings while unrelated availability or deprecation diagnostics remain intact.

Written by the indexing model from the issue text.