[perl #39714] [TODO] Refactor IMCC to remove static globals

[Audrey Tang]

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IMCC currently relies on a lot of static globals to carry state, and
cannot reliably restore them when an error occurs. (grep for
"static" and "FIXME global" in the IMCC tree.)

Allison had ruled that reentrancy should be possible for IMCC, and
this would be a good refactoring project.