Rename the non-templated base class of SmallPtrSet to

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200687 91177308-0d34-0410-b5e6-96231b3b80d8