[memprof] Add Version2 of the indexed MemProf format (#89100)

This patch adds Version2 of the indexed MemProf format.  The new
format comes with a hash table from CallStackId to actual call stacks
llvm::SmallVector<FrameId>.  The rest of the format refers to call
stacks with CallStackId.  This "values + references" model effectively
deduplicates call stacks.  Without this patch, a large indexed memprof
file of mine shrinks from 4.4GB to 1.6GB, a 64% reduction.

This patch does not make Version2 generally available yet as I am
planning to make a few more changes to the format.