[MemProf][NFC] Clear each IndexedMemProfRecord after it is written (#75205)

The on-disk hash table for the memprof writer holds copies of all the
memprof records to be written. These hold a lot of memory in aggregate,
due to the lists of alloc sites (which each have a list of context
frames) and call sites. Clear each one after emitting it.

This drops the peak memory when writing a very large indexed memprof
profile by about 2.5G.