Rollup merge of #63955 - RalfJung:intern, r=oli-obk

Make sure interned constants are immutable

This makes sure that interning for constants (not statics) creates only immutable allocations.

Previously, the "main" allocation of `const FOO: Cell<i32> = Cell::new(0);` was marked as mutable, but I don't think we want that. It can be only copied, not written to.

Also, "leftover" allocations (behind raw pointers etc) were left mutable. I don't think we want to support that. I tried asserting that these are all already immutable (to double-check our static checks), but that failed in this one:
```rust
const NON_NULL_PTR2: NonNull<u8> = unsafe { mem::transmute(&0) };
```
Seems like maybe we want more precise mutability annotation inside Miri for locals (like `&0` here) so that this would actually become immutable to begin with?

I also factored `intern_shallow` out of the visitor so that we don't have to construct a visitor when we do not plan to visit anything. That confused me at first.

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