Add Miri support for move elimination semantics

[Amanieu]

Adds support to Miri for the new MIR semantics from RFC 3943 when -Zmir-move-elimination is enabled. This builds on top of #157943, look at the last commit only.

Notable changes:

• Added a LocalValue::LiveUnallocated state to represent a local that is StorageLive but hasn't been eliminated yet.
  • Except for ZSTs: those are always fully allocated at StorageLive. This is necessary for the RemoveZsts pass and because several MIR constructors leave ZST places uninitialized.
• Added eval_place_for_write which allocates a local if it is currently LiveUnallocated.
• Move operands with a bare local are evaluated by move_out_local: it copies the value to a temporary allocation freed at the end of the statement, and then frees the original allocation, switching the local to LiveUnallocated.
• FnArg::InPlace arguments that are bare locals are recorded as part of the ReturnContinuation. They are freed upon returning or unwinding back to the caller.
• Rvalue evaluation needs to evaluate the destination after all operands. This is done by evaluating the rvalue into a temporary allocation, and then moving that to the evaluated destination.

r? @ghost

[rustbot]

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[Amanieu]

Oops, that should've been a draft.

[rust-bors]

☔ The latest upstream changes (presumably #160524) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.