[MemProf][NFC] Free large data structures after last use (#75120)

The MemProf InstrProfWriter uses a couple of MapVector for building the
lists of records it needs to write. Once its entries are all added to
the associated OnDiskChainedHashTableGenerator, it is no longer used.

Clearing these MapVectors, which grow quite large for large profiles,
saved 4G for a large memory profile.