Fix #124478 - offset_of! returns a temporary

This was due to the must_use() call. Adding HIR's OffsetOf to the must_use
checking within the compiler avoids this issue.

compiler/rustc_lint/src/unused.rs

@@ -176,6 +176,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 | hir::BinOpKind::Shr => Some("bitwise operation"),
             },
             hir::ExprKind::AddrOf(..) => Some("borrow"),
+            hir::ExprKind::OffsetOf(..) => Some("`offset_of` call"),
             hir::ExprKind::Unary(..) => Some("unary operation"),
             _ => None,
         };