Perform OpaqueCast field projection on HIR, too.

This is necessary for closure captures in 2021 edition, as they capture individual fields, not the full mentioned variables. So it may try to capture a field of an opaque (because the hidden type is known to be something with a field).

compiler/rustc_middle/src/ty/closure.rs

@@ -174,6 +174,8 @@ impl<'tcx> CapturedPlace<'tcx> {
                 // Ignore derefs for now, as they are likely caused by
                 // autoderefs that don't appear in the original code.
                 HirProjectionKind::Deref => {}
+                // Just change the type to the hidden type, so we can actually project.
+                HirProjectionKind::OpaqueCast => {}
                 proj => bug!("Unexpected projection {:?} in captured place", proj),
             }
             ty = proj.ty;