Fix UndefRefError from LoweredCodeUtils from InfiniteMPS method signatures

[borisdevos]

Description

No clue what the origin of the issue was, so I asked my robot friend for a fix and a brief explanation. Here me paraphrasing it: "Writing S <: Union{Int, <:IndexSpace} puts an anonymous type variable inside the bound of S. For methods with keyword arguments, Revise (through LoweredCodeUtils) re-runs the method's signature to find the generated keyword-body function. It doesn't handle that inner type variable correctly, fails with UndefRefError, and skips the method with a warning. Union{Int, <:IndexSpace} == Union{Int, IndexSpace}, so writing S <: Union{Int, IndexSpace} defines the same methods and avoids the warning."

This would occasionally freeze my REPL in some Revise retrigger, which was very annoying.

Checklist

• Tests pass locally (julia --project=test test/runtests.jl, or the relevant subset)
• Documentation updated, if this PR changes public API (docstrings, docs/src/)
• Runic formatter is run
• Changelog entry added under [Unreleased] in docs/src/changelog.md, if this PR is user-facing (new feature, behavior change, bug fix, deprecation, or removal)

[lkdvos]

fair enough, can you also open an issue for Revise itself?