layers: Use accessible id set when determining shader's descriptor use

layers: Use accessible id set when determining shader's descriptor use

- Eliminates bogus false conflicts between resources in a module, where the resources share a descriptor slot but
  no entrypoint refers to both.
- Eliminates false positives from shader vs descriptor set layout validation, where we would previously be upset
  about the layout not including a descriptor which was declared in the module but not used by the entrypoint.

Signed-off-by: Chris Forbes <chrisforbes@google.com>