Combine `HaveBeenBorrowedLocals` and `IndirectlyMutableLocals` into one dataflow analysis
This PR began as an attempt to port `HaveBeenBorrowedLocals` to the new dataflow framework (see #68241 for prior art). Along the way, I noticed that it could share most of its code with `IndirectlyMutableLocals` and then found a few bugs in the two analyses:
- Neither one marked locals as borrowed after an `Rvalue::AddressOf`.
- `IndirectlyMutableLocals` was missing a minor fix that `HaveBeenBorrowedLocals` got in #61069. This is not a problem today since it is only used during const-checking, where custom drop glue is forbidden. However, this may change some day.
I decided to combine the two analyses so that they wouldn't diverge in the future while ensuring that they remain distinct types (called `MaybeBorrowedLocals` and `MaybeMutBorrowedLocals` to be consistent with the `Maybe{Un,}InitializedPlaces` naming scheme). I fixed the bugs and switched to exhaustive matching where possible to make them less likely in the future. Finally, I added comments explaining some of the finer points of the transfer function for these analyses (see #61069 and #65006).
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