Auto merge of #114208 - GKFX:offset_of_enum, r=wesleywiser

Support enum variants in offset_of!

This MR implements support for navigating through enum variants in `offset_of!`, placing the enum variant name in the second argument to `offset_of!`. The RFC placed it in the first argument, but I think it interacts better with nested field access in the second, as you can then write things like

```rust
offset_of!(Type, field.Variant.field)
```

Alternatively, a syntactic distinction could be made between variants and fields (e.g. `field::Variant.field`) but I'm not convinced this would be helpful.

[RFC 3308 # Enum Support](https://rust-lang.github.io/rfcs/3308-offset_of.html#enum-support-offset_ofsomeenumstructvariant-field_on_variant)
Tracking Issue #106655.

compiler/stable_mir/src/mir/body.rs

@@ -516,7 +516,7 @@ pub enum NullOp {
     /// Returns the minimum alignment of a type.
     AlignOf,
     /// Returns the offset of a field.
-    OffsetOf(Vec<FieldIdx>),
+    OffsetOf(Vec<(VariantIdx, FieldIdx)>),
 }
 
 impl Operand {