[memprof] Use const ref for IndexedRecord (#94114)

The type of *Iter here is "const IndexedMemProfRecord &" as defined in
RecordLookupTrait.  Assigning *Iter to a variable of type
"const IndexedMemProfRecord &" avoids a copy, reducing the cycle and
instruction counts by 1.8% and 0.2%, respectively, with
"llvm-profdata show" modified to deserialize all MemProfRecords.

Note that RecordLookupTrait has an internal copy of
IndexedMemProfRecord, so we don't have to worry about a dangling
reference to a temporary.