invoked calls: record invoke signature in backedges (#46010)

This includes only the changes to `dump.c` for this change, but excludes
the functional part of the change (except for the additional bugfixes
mentioned below).

ORIGINAL COMMIT TEXT:
    This fixes a long-standing issue with how we've handled `invoke` calls
    with respect to method invalidation.  When we load a package, we need
    to ask whether a given MethodInstance would be compiled in the same
    way now (aka, in the user's running session) as when the package was
    precompiled; in practice, the way we do that is to test whether the
    dispatches would be to the same methods in the current
    world-age. `invoke` presents special challenges because it allows the
    coder to deliberately select a different method than the one that
    would be chosen by ordinary dispatch; if there is no record of how
    this choice was made, it can look like it resolves to the wrong method
    and this can trigger invalidation.

    This allows a MethodInstance to store dispatch tuples as well as other
    MethodInstances among their backedges.

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.

Co-authored-by: Jameson Nash <[email protected]>

(cherry picked from commits dd375e1,
cb0721b, 9e39fe9)