Remove SmallMap, and the several files that were used to implement it.

We have SmallDenseMap now that has more correct and predictable
semantics, even though it is a more narrow abstraction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158644 91177308-0d34-0410-b5e6-96231b3b80d8