[instrprof][NFC] Templatize the instrprof iterator.

This change templatizes the InstrProfIterator where the default
specialization is based on the current usage, i.e. the reader_type is
InstrProfReader and the record_type (value_type) is
NamedInstrProfRecord. A subsequent patch will use the same iterator
template to implement an iterator for the RawMemProfReader.

Differential Revision: https://reviews.llvm.org/D116782