Mark generic arguments for unresolved generic types

[sbomer]

Fixes an ILLink crash when a base generic type is not resolvable. See #93797 (comment) for context. The failure mode was this:

• A generic type isn't resolvable because it's defined in an unreferenced assembly
• But the generic arguments are resolvable. Before the fix, we wouldn't necessarily mark these when the generic type isn't resolvable.
• When sweeping the assembly, we drop the typeref for the unmarked generic argument type.
• When writing the assembly, we try to write out the generic type reference, but crash because the generic argument type doesn't exist anymore.

This fixes it by marking the generic argument types even when the generic type definition can't be resolved.

Fixes #93797

@dotnet/illink

[jtschuster]

LGTM, thank you!