[ADT] Rename NumNonEmpty to NumEntries in SmallPtrSet (NFC) (#153757)
Without this patch, we use NumNonEmpty, which keeps track of the
number of valid entries plus tombstones even though we have a separate
variable to keep track of the number of tombstones.
This patch simplifies the metadata. Specifically, it changes the name
and semantics of the variable to NumEntries to keep track of the
number of valid entries.
The difference in semantics requires some code changes aside from
mechanical replacements:
- size() just returns NumEntries.
- erase_imp() and remove_if() need to decrement NumEntries in the
large mode.
- insert_imp_big() increments NumEntries for successful insertions,
regardless of whether a tombstone is being replaced with a valid
entry. It also computes the number of non-tombstone empty slots as:
CurArraySize - NumEntries - NumTombstones
- Grow() no longer needs NumNonEmpty -= NumTombstones.
Overall, the resulting code should look more intuitive and more
consistent with DenseMapSet.