[mlir][AttrType] Emit unbalanced character errors using the last punctuation used

This gives a better diagnostic in general, because it indicates that the user didn't
close out the last open puncutation range. For example:

```
foo.op {
  some.op -> !blah.pointer<
}
```
We want the error to hint about the unclosed `<`, not the `}` (which isn't really in the context of the type).

Differential Revision: https://reviews.llvm.org/D141635