[ADT] Fix an indexing bug in PackedVector (#158785)

PackedVector is like std::vector<int> except that we can store small
elements (e.g. 2-bit elements) in a packed manner using a BitVector as
the underlying storage.

The problem is that for bit size 3 and beyond, the calculation of
indices into the underlying BitVector is not correct.  For example,
around line 50, we see a "for" loop to retrieve an unsigned integer
value:

  for (unsigned i = 0; i != BitNum-1; ++i)
    val = T(val | ((Bits[(Idx << (BitNum-1)) + i] ? 1UL : 0UL) << i));

Suppose that BitNum is 4 (that is, 4-bit item).  Here is the mapping
between the PackedVector index and the corresponding BitVector
indices.

  Idx 0:  0, 1, 2, 3
  Idx 1:  8, 9, 10, 11
  Idx 2:  16, 17, 18, 19

That is, we use 4 bits out of every 8 bits.  This is because the index
calculation uses "<<".  The index should really be Idx * BitNum + i.
FWIW, all the methods in PackedVector consistently use the shift-based
index calculation, so the user would never encounter a bug except
possibly as excessive storage use.

This patch fixes the index calculation.  Now, in size(), I didn't want
to do integer division:

  return Bits.size() / BitNum;

so this patch adds a separate variable NumElements to keep track of
the number of elements.

The unit test checks for the expected size of the underlying
BitVector.