[memprof] Remove const from the return type of toMemProfRecord (#93415)

"const" being removed in this patch prevents the move semantics from
being used in:

  AI.CallStack = Callback(IndexedAI.CSId);

With this patch on an indexed MemProf Version 2 profile, the cycle
count and instruction count go down by 13.3% and 26.3%, respectively,
with "llvm-profdata show" modified to deserialize all MemProfRecords.