For each captured region of this alias, compute the captured identity
args that are known to outlive it, given the definition of the opaque type
in the the parent context.
For a given alias type, this returns the set of (identity) generic args that
are relevant for liveness, that can be inferred from outlives bounds on the
alias itself, and the explicit and implicit outlives clauses of the alias.
Callers should instantiate the returned args with the concrete args of the alias.
For a param-env clause for<'v..> <T as Trait>::Assoc<..>: 'bound that
applies to ty (an alias with alias_def_id), returns the set of (identity) args
that the underlying type could possibly capture, as restricted by this clause.