[ProfileData] Use std::vector for ValueData (NFC) (#95194)

This patch changes the type of ValueData to
std::vector<InstrProfValueData> so that, in a follow-up patch, we can
teach getValueForSite to return ArrayRef<InstrProfValueData>.

Currently, a typical traversal over the value data looks like:

  uint32_t NV = Func.getNumValueDataForSite(VK, I);
std::unique_ptr<InstrProfValueData[]> VD = Func.getValueForSite(VK, I);
  for (uint32_t V = 0; V < NV; V++)
    Do something with VD[V].Value and/or VD[V].Count;

Note that we need to call getNumValueDataForSite and getValueForSite
separately.  If getValueForSite returns ArrayRef<InstrProfValueData>
in the future, then we'll be able to do something like:

  for (const auto &V : Func.getValueForSite(VK, I))
    Do something with V.Value and/or V.Count;

If ArrayRef<InstrProfValueData> directly points to ValueData, then
getValueForSite won't need to allocate memory with std::make_unique.

Now, switching to std::vector requires us to update several places:

- sortByTargetValues switches to llvm::sort because we don't need to
  worry about sort stability.

- sortByCount retains sort stability because std::list::sort also
  performs stable sort.

- merge builds another array and move it back to ValueData to avoid a
  potential quadratic behavior with std::vector::insert into the
  middle of a vector.