Tweak the way we protect in-place function arguments in interpreters

Use `MPlaceTy` instead of `PlaceTy` in `FnArg` and ignore (copy) locals in an
earlier step ("Locals that don't have their address taken are as protected as
they can ever be").

This seems to be crucial for tail call support (as they can't refer to caller's
locals which are killed when replacing the stack frame).

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -227,7 +227,7 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
         if self.tcx.has_attr(def_id, sym::rustc_const_panic_str)
             || Some(def_id) == self.tcx.lang_items().begin_panic_fn()
         {
-            let args = self.copy_fn_args(args)?;
+            let args = self.copy_fn_args(args);
             // &str or &&str
             assert!(args.len() == 1);
 
@@ -254,7 +254,7 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
 
             return Ok(Some(new_instance));
         } else if Some(def_id) == self.tcx.lang_items().align_offset_fn() {
-            let args = self.copy_fn_args(args)?;
+            let args = self.copy_fn_args(args);
             // For align_offset, we replace the function call if the pointer has no address.
             match self.align_offset(instance, &args, dest, ret)? {
                 ControlFlow::Continue(()) => return Ok(Some(instance)),