[msl] Fix read-write textureStore in entry points

When emitting calls to the `fence()` builtin member function, wrap the
object in a `const_cast<>` to remove the `const` qualifier.

We do this to work around an MSL bug that prevents us from being able
to use texture fence intrinsics when texture handles are stored inside
const-qualified structures.

Fixed: 365570202
Change-Id: I8571804f063e60f544e2150f1454a6d7c63a4af3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/206194
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: James Price <jrprice@google.com>