rename BorrowKind::Shallow to Fake

also adds some comments

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -456,7 +456,7 @@ impl<'tcx> Validator<'_, 'tcx> {
         match kind {
             // Reject these borrow types just to be safe.
             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
-            BorrowKind::Shallow | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
+            BorrowKind::Fake | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
                 return Err(Unpromotable);
             }