[memprof] Use CSId to construct MemProfRecord (#88362)

We are in the process of referring to call stacks with CallStackId in
IndexedMemProfRecord and IndexedAllocationInfo instead of holding call
stacks inline (both in memory and the serialized format).  Doing so
deduplicates call stacks and reduces the MemProf profile file size.

Before we can eliminate the two fields holding call stacks inline:

- IndexedAllocationInfo::CallStack
- IndexedMemProfRecord::CallSites

we need to eliminate all the read operations on them.

This patch is a step toward that direction.  Specifically, we
eliminate the read operations in the context of MemProfReader and
RawMemProfReader.  A subsequent patch will eliminate the read
operations during the serialization.