[ADT] Simplify SmallPtrSetIterator with std::reverse_iterator (NFC) (#160643)

SmallPtrSetIterator has two tasks:

 - iterate the buckets in the requested direction
 - skip the empty and tombstone buckets

These tasks are intertwined in the current implementation. This patch
separates them.

A new private iterator type, BucketItTy, now handles the iteration
direction. This is an alias for a raw pointer for forward iteration
and std::reverse_iterator<pointer> for reverse iteration.

The user-facing iterator now focuses solely on advancing BucketItTy
while skipping invalid (empty or tombstone) buckets. AdvanceIfNotValid
now works transparently for both directions. operator++ on BucketItTy
does the right thing whether it's a raw pointer or a
std::reverse_iterator.

This simplification removes RetreatIfNotValid and the
reverse-iteration logic in operator*() and operator++().

0 files changed, 0 insertions, 0 deletions