[memprof] Add another constructor to MemProfReader (#88952)

This patch enables users of MemProfReader to directly supply mappings
from CallStackId to actual call stacks.

Once the users of the current constructor without CSIdMap switch to
the new constructor, we'll have fewer users of:

- IndexedAllocationInfo::CallStack
- IndexedMemProfRecord::CallSites

bringing us one step closer to the removal of these fields in favor
of:

- IndexedAllocationInfo::CSId
- IndexedMemProfRecord::CallSiteIds