Update bounds checking notes

[kkjeer]

This PR makes a minor modification to the notes emitted during bounds checking:

Only one note is emitted per statement per lvalue expression for an expression with unknown bounds being assigned to an lvalue expression.
For example:

void f(_Array_ptr<int> p : count(1), _Array_ptr<int> q : bounds(unknown) {
  p = q, p++;
}

Both assignments p = q and p++ assign expressions (q and p + 1 respectively) with unknown bounds to p. A note will only be emitted for the first assignment. This helps avoid confusion in the note messages - it may be unclear to the user why the expression p++ would result in the note "assigned expression p + 1 with unknown bounds to p".

[sulekhark]

LGTM! Thank you!