Make missing_const_for_fn operate on non-optimized MIR

This has two reasons: First of all we don't need the optimized MIR for
evaluating the checks of this lint, as const-eval anyways operates on
`mir_for_ctf` which is derived from `mir_drops_elaborated_and_const_checked`.
Second of all we might transform MIR in the optimization passes in a way
that doesn't work with const-eval, but it is irrelevant since const-eval
uses another MIR.

Specifically this came up when adding a new check in debug builds
(rust-lang/rust#134424), which is added as part
of an optimization pass.

Author: 1c3t3a

clippy_lints/src/missing_const_for_fn.rs

@@ -157,9 +157,9 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
             return;
         }
 
-        let mir = cx.tcx.optimized_mir(def_id);
+        let mir = cx.tcx.mir_drops_elaborated_and_const_checked(def_id);
 
-        if let Ok(()) = is_min_const_fn(cx.tcx, mir, &self.msrv)
+        if let Ok(()) = is_min_const_fn(cx.tcx, &mir.borrow(), &self.msrv)
             && let hir::Node::Item(hir::Item { vis_span, .. }) | hir::Node::ImplItem(hir::ImplItem { vis_span, .. }) =
                 cx.tcx.hir_node_by_def_id(def_id)
         {