Add a new warning, -Wlogical-not-parentheses, to -Wparentheses.

This warning triggers on the logical not of a non-boolean expression on the
left hand side of comparison.  Often, the user meant to negate the comparison,
not just the left hand side of the comparison.  Two notes are also emitted,
the first with a fix-it to add parentheses around the comparison, and the other
to put parenthesis around the not expression to silence the warning.

bool not_equal(int x, int y) {
  return !x == y;  // warn here
}

  return !(x == y);  // first fix-it, to negate comparison.

  return (!x) == y;  // second fix-it, to silence warning.