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_ty_utils/src/consts.rs

@@ -256,6 +256,7 @@ fn recurse_build<'tcx>(
         ExprKind::VarRef { .. }
         | ExprKind::UpvarRef { .. }
         | ExprKind::StaticRef { .. }
+        | ExprKind::OffsetOf { .. }
         | ExprKind::ThreadLocalRef(_) => {
             error(GenericConstantTooComplexSub::OperationNotSupported(node.span))?
         }
@@ -347,6 +348,7 @@ impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
             | thir::ExprKind::ZstLiteral { .. }
             | thir::ExprKind::StaticRef { .. }
             | thir::ExprKind::InlineAsm(_)
+            | thir::ExprKind::OffsetOf { .. }
             | thir::ExprKind::ThreadLocalRef(_)
             | thir::ExprKind::Yield { .. } => false,
         }