[ADT] Avoid map storage for small SmallMapVector (#196473)
SmallMapVector previously used SmallDenseMap for its index, which still
initializes and maintains map storage even when the number of entries is
tiny.
Teach MapVector to support a vector-only small mode. While the entry
count stays
within the configured small size, operations use the underlying vector
directly.
When the size grows past the threshold, the map index is built and
subsequent
operations use the regular MapVector path.
This mirrors the small-size strategy used by SmallSetVector.
