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.

llvm-svn: 200687