This patch refactors getMemProfRecord for readability while adding
consistency checks.
- This patch adds a switch statement on the MemProf version just like
most places dealing with MemProf serialization/deserialization.
- This patch adds asserts to ensure that the exact set of data
structures are available while ones we do not use are not present.
That is, getMemProfRecord no longer determines the version based on
the availability of MemProfCallStackTable.
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