Rollup merge of #144912 - LorrensP-2158466:smart-resolver, r=petrochenkov

Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.

This pr introduces a `CmResolver`, a wrapper around the main resolver which gives out mutable access given a condition.

`CmResolver` only allows mutation when we’re not in speculative import resolution. This ensures we can’t accidentally mutate the resolver during this process, which is important as we move towards a batched resolution algorithm.

This also changes functions that are used during speculative import resolution to take a `CmResolver` instead of a `&mut Resolver`.

Also introduces a new kind of "smart pointer" which has the behaviour described above:
```rust
/// A wrapper around a mutable reference that conditionally allows mutable access.
pub(crate) struct RefOrMut<'a, T> {
    p: &'a mut T,
    mutable: bool,
}

type CmResolver<'r, 'ra, 'tcx> = RefOrMut<'r, Resolver<'ra, 'tcx>>;
```
r? petrochenkov

Author: Zalathar