[memprof] Add MemProf format Version 3 (#93608)

This patch adds Version 3 for development purposes.  For now, this
patch adds V3 as a copy of V2.

For the most part, this patch adds "case Version3:" wherever "case
Version2:" appears.  One exception is writeMemProfV3, which is copied
from writeMemProfV2 but updated to write out memprof::Version3 to the
MemProf header.  We'll incrementally modify writeMemProfV3 in
subsequent patches.