[memprof] Introduce handleCallSite (NFC) (#149724)
Continuing the effort to refactor readMemProf, this patch introduces
handlCallSite to handle, well, call sites.
Moving the code requires taking CallSiteEntry and CallSiteEntryHash
out of readMemProf.
We could simplify some code, but I'm keeping this patch very simple to
facilitate the review process. For example, we could simplify the
control flow near the end of readMemProf, but we can address that
later.
