* Functions failing codegen now set this failure on the function
  analysis state. Decl analysis `codegen_failure` is reserved for
  failures generating constant values.

* `liveness_failure` is consolidated into `codegen_failure`, as we do
  not need to distinguish these, and Liveness.Verify is just a debugging
  feature anyway.

* `sema_failure_retryable` and `codegen_failure_retryable` are removed.
  Instead, retryable failures are recorded in the new
  `Zcu.retryable_failures` list. On an incremental update, this list is
  flushed, and all elements are marked as outdated so that we re-attempt
  analysis and code generation.

Also remove the `generation` fields from `Zcu` and `Decl` as these are
not needed by our new strategy for incremental updates.