Add a SmallBitVector class, which mimics BitVector but uses only

Add a SmallBitVector class, which mimics BitVector but uses only
a single pointer (PointerIntPair) member. In "small" mode, the
pointer field is reinterpreted as a set of bits. In "large" mode,
the pointer points to a heap-allocated object.

Also, give BitVector empty and swap functions.

And, add some simple unittests for BitVector and SmallBitVector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92730 91177308-0d34-0410-b5e6-96231b3b80d8