Auto merge of #106934 - DrMeepster:offset_of, r=WaffleLapkin

Add offset_of! macro (RFC 3308)

Implements https://github.com/rust-lang/rfcs/pull/3308 (tracking issue #106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

compiler/rustc_mir_transform/src/separate_const_switch.rs

@@ -303,8 +303,7 @@ fn find_determining_place<'tcx>(
                     | Rvalue::NullaryOp(_, _)
                     | Rvalue::ShallowInitBox(_, _)
                     | Rvalue::UnaryOp(_, Operand::Constant(_))
-                    | Rvalue::Cast(_, Operand::Constant(_), _)
-                    => return None,
+                    | Rvalue::Cast(_, Operand::Constant(_), _) => return None,
                 }
             }