Stacked Borrows retags should also be accesses for the data race model

[RalfJung]

Currently, many Stacked Borrows retags are effectively read/write accesses for the aliasing model:

miri/src/stacked_borrows/mod.rs

Lines 508 to 511 in 9568d7e

	// A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
	// Here, creating a reference actually counts as an access.
	// This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
	self.access(access, derived_from, global, dcx, exposed_tags)?;

However, the data race model never hears about them, which as @JakobDegen points out leads us to accept some code that we probably should not accept.