[memprof] Group MemProf data structures into a struct (NFC) (#92360)

This patch groups the three Memprof data structures into a struct
named IndexedMemProfData and teaches InstrProfWriter to use it.  This
way, we can pass IndexedMemProfData to writeMemProf and its helpers
instead of individual data structures.

As a follow-up, we can use the new struct in MemProfReader also.  That
in turn allows loadInput in llvm-profdata to move the MemProf data
into the writer context, saving a few seconds for a large MemProf
profile.