[SmallPtrSet] Introduce a find primitive and rewrite count/erase in terms of it

This was originally motivated by a compile time problem I've since figured out how to solve differently, but the cleanup seemed useful. We had the same logic - which essentially implemented find - in several places. By commoning them out, I can implement find and allow erase to be inlined at the call sites if profitable.

Differential Revision: https://reviews.llvm.org/D28183

llvm-svn: 290779