Rename the non-templated base class of SmallPtrSet to
'SmallPtrSetImplBase'. This more closely matches the organization of
SmallVector and should allow introducing a SmallPtrSetImpl which serves
the same purpose as SmallVectorImpl: isolating the element type from the
particular small size chosen. This in turn allows a lot of
simplification of APIs by not coding them against a specific small size
which is rarely needed.
Introduce SmallPtrSetImpl<T *> which allows insert, erase, count, and
iteration. This alows the majority of operations to be performed without
encoding a specific small size. It follows the model of
SmallVectorImpl<T>.