This allows a MethodInstance to store dispatch tuples as well as other
MethodInstances among their backedges. The motivation is to properly
handle invalidation with `invoke`, which allows calling a
less-specific method and thus runs afoul of our standard mechanisms to
detect "outdated" dispatch.

Additionally:

- provide backedge-iterators for both C and Julia that abstracts
  the specific storage mechanism.

- fix a bug in the CodeInstance `relocatability` field, where methods
  that only return a constant (and hence store `nothing` for
  `inferred`) were deemed non-relocatable.

- fix a bug in which #43990 should have checked that the method had
  not been deleted. Tests passed formerly simply because we weren't
  caching external CodeInstances that inferred down to a `Const`;
  fixing that exposed the bug.  This bug has been exposed since
  merging #43990 for non-`Const` inference, and would affect Revise
  etc.

This mimics an `invoke` call

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Encode the `invokesig` in `ext_targets` so it gets cached to disk.