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.

llvm-svn: 92730