[memprof] Introduce writeMemProf (NFC) (#87698)

This patch refactors the serialization of MemProf data to a switch
statement style:

  switch (Version) {
  case Version0:
    return ...;
  case Version1:
    return ...;
  }

just like IndexedMemProfRecord::serialize.

A reasonable amount of code is shared and factored out to helper
functions between writeMemProfV0 and writeMemProfV1 to the extent that
doens't hamper readability.