don't UB on dangling ptr deref, instead check inbounds on projections
This implements https://github.com/rust-lang/reference/pull/1387 in Miri. See that PR for what the change is about.
Detecting dangling references in `let x = &...;` is now done by validity checking only, so some tests need to have validity checking enabled. There is no longer inherently a "nodangle" check in evaluating the expression `&*ptr` (aside from the aliasing model).
r? `@oli-obk`
Based on:
- https://github.com/rust-lang/reference/pull/1387
- https://github.com/rust-lang/rust/pull/115524
e7bdc5f9f8