Basic: Use APInt to implement ClusteredBitVector
Simplify the implementation of ClusteredBitVector by using an APInt
to represent the raw bits. This simplification will make it easier
to incrementally move to a representation of bit vectors that works
on both big- and little-endian machines.
This commit also removes reserve and reserveExtra from the API
since they were only used in one place and no longer have any effect
because memory allocation is now handled by the APInt class.
