Improve information sharing across SB diagnostics

Previous Stacked Borrows diagnostics were missing a lot of information
about the state of the interpreter, and it was difficult to add
additional state because it was threaded through all the intervening
function signatures.

This change factors a lot of the arguments which used to be passed
individually to many stacked borrows functions into a single
`DiagnosticCx`, which is built in `Stacks::for_each`, and since it
wraps a handle to `AllocHistory`, we can now handle more nuanced
things like heterogeneous borrow of `!Freeze` types.

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