Update {Small}BitVector size_type definition

SmallBitVector implements a level of indirection over BitVector by
storing a smaller bit-vector in a pointer-sized element, or in case the
number of elements exceeds the bucket size, it creates a new pointer to
a BitVector and uses that as its storage.

However, the functions returning the vector size were using `unsigned`,
which is ok for BitVector, but not for SmallBitVector, which is actually
`uintptr_t`.

This commit reuses the `size_type` definition to more than just `count`
and propagates them into range iteration, size calculation, etc.

This is a continuation of D108124.

I haven't changed all occurrences of `unsigned` or `uintptr_t` to
`size_type`, just those that were directly related.

Following directions from clang-tidy on case of variables.

Differential Revision: https://reviews.llvm.org/D108290