[memprof] Simplify readMemprof (NFC) (#119930)
This patch essentially replaces:
std::pair<const std::vector<Frame> *, unsigned>
with:
ArrayRef<Frame>
This way, we can store and pass ArrayRef<Frame>, conceptually one
item, instead of the pointer and index.
The only problem is that we don't have an existing hash function for
ArrayRef<Frame>>, so we provide a custom one, namely
CallStackHash.
