Auto merge of #148151 - cjgillot:no-offset-of, r=scottmcm

Replace OffsetOf by an actual sum of calls to intrinsic.

This PR changes the way we compute the value of the `offset_of!` macro in MIR. The current implementation uses a dedicated MIR rvalue.

This PR proposes to replace it by an inline constant which sums calls to a new intrinsic `offset_of(variant index, field index)`. The desugaring is done at THIR building time, easier that doing it on MIR.

The new intrinsic is only meant to be used by const-eval. LLVM codegen will refuse to generate code for it.

We replace:
```rust
a = offset_of!(T, Variant1.Field1.Variant2.Field2);
```

By:
```rust
a = const {constant#n};

{constant#n}: usize = {
    _1 = offset_of::<T>(index of Variant1, index of Field1);
    _2 = offset_of::<U>(index of Variant2, index of Field2); // Where T::Variant1::Field1 has type U
    _0 = _1 + _2
}
```

The second commit modifies intrinsic const checking to take `allow_internal_unstable` into account. The new intrinsic should only be called from stable `offset_of!` macro. The intrinsic itself is unstable, const-unstable, but `rustc_intrinsic_const_stable_indirect`.

Fixes rust-lang/rust#123959
Fixes rust-lang/rust#125680
Fixes rust-lang/rust#129425
Fixes rust-lang/rust#136175

r? `@ghost`

Author: bors