Bug: handling of indexing in unsafe code is unsound

[Nadrieril]

Currently we translate a Index place expression to a function call on a borrow of that place. This is not-strictly-correct wrt retags for safe code, and actually unsound if the access is behind a raw pointer.

We should only do this for references, and put that behavior behind a flag, along with other reconstructions we do that don't make sense for users who want precisely accurate MIR.