[memprof] Add MemProfReader::takeMemProfData (#116769)

This patch adds MemProfReader::takeMemProfData, a function to return
the complete MemProf profile from the reader.  We can directly pass
its return value to InstrProfWriter::addMemProfData without having to
deal with the indivual components of the MemProf profile.  The new
function is named "take", but it doesn't do std::move yet because of
type differences (DenseMap v.s. MapVector).

The end state I'm trying to get to is roughly as follows:

- MemProfReader accepts IndexedMemProfData as a parameter as opposed
  to the three individual components (frames, call stacks, and
  records).

- MemProfReader keeps IndexedMemProfData as a class member without
  decomposing it into its individual components.

- MemProfReader returns IndexedMemProfData like:

  IndexedMemProfData takeMemProfData() {
    return std::move(MemProfData);
  }