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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> Do you think we should add the v2 reader to the same patch so it can be tested as a round trip? Right now this patch doesn't have any tests.

The very first iteration has a smoke test `llvm/test/tools/llvm-profdata/memprof-merge-v0.test`, which is very weak.

The latest iteration adds a roundtrip unit test to `llvm/unittests/ProfileData/InstrProfTest.cpp`.  Please take a look.  Thanks!

https://github.com/llvm/llvm-project/pull/89100