Don't allow multi-step dependencies in caches

[Tom de Vries]

Hi,

this patch series implements the forbidding of multi-step garbage 
collection liveness dependencies between caches.

The first four patches downgrade 3 caches to non-cache, since they 
introduce multi-step dependencies. This allows us to decouple:
- establishing a policy for multi-step dependencies in caches, and
- fixing issues that allow us to use these 3 as caches again.

1. Downgrade debug_args_for_decl to non-cache
2. Add struct tree_decl_map_hasher
3. Downgrade debug_expr_for_decl to non-cache
4. Downgrade value_expr_for_decl to non-cache
5. Don't mark live recursively in gt_cleare_cache

Bootstrapped and reg-tested on x86_64, with ENABLE_CHECKING.

I'll post the patches in response to this email.

Thanks,
- Thanks