[Clang] Diagnose invalid pointer overflow check with intermediate variable

[nikic]

#120222 added support to -Wtautological-compare to diagnose comparisons like ptr + unsigned_offset < ptr, which are always false due to undefined behavior on pointer overflow during addition.

However, most commonly this pattern appears when the result of the addition is stored in an intermediate variable first (because it will also be used later):

bool test(const char *ptr, size_t index) {
  const char *end_ptr = ptr + index;
  return end_ptr < ptr;
}

It would be great to diagnose these cases as well, using an analysis-based warning.

cc @AaronBallman @ldionne as we discussed this on Wednesday.

[AaronBallman]

CC @devincoughlin -- I was thinking we could perhaps add an analysis-based warning (not static analyzer) to catch this; is this something your team might be interested in helping with given that it's somewhat related to bounds safety?