[memprof] Add InstrProfWriter::addMemProfData (#116528)

This patch adds InstrProfWriter::addMemProfData, which adds the
complete MemProf profile (frames, call stacks, and records) to the
writer context.

Without this function, functions like loadInput in llvm-profdata.cpp
and InstrProfWriter::mergeRecordsFromWriter must add one item (frame,
call stack, or record) at a time.  The new function std::moves the
entire MemProf profile to the writer context if the destination is
empty, which is the common use case.  Otherwise, we fall back to
adding one item at a time behind the scene.

Here are a couple of reasons why we should add this function:

- We've had a bug where we forgot to add one of the three data
  structures (frames, call stacks, and records) to the writer context,
  resulting in a nearly empty indexed profile.  We should always
  package the three data structures together, especially on API
  boundaries.

- We expose a little too much of the MemProf detail to
  InstrProfWriter.  I'd like to gradually transform
  InstrProfReader/Writer to entities managing buffers (sequences of
  bytes), with actual serialization/deserialization left to external
  classes.  We already do some of this in InstrProfReader, where
  InstrProfReader "contracts out" to IndexedMemProfReader to handle
  MemProf details.

I am not changing loadInput or InstrProfWriter::mergeRecordsFromWriter
for now because MemProfReader uses DenseMap for frames and call
stacks, whereas MemProfData uses MapVector.  I'll resolve these
mismatches in subsequent patches.