[flang] Replay a FORMAT at the right position

When FORMAT control reaches the final parenthesis and data items
remain, we advance a record and revert to the beginning of the
FORMAT for further items.  But when the FORMAT contains any
nested parenthesized group of editing descriptors, possibly
repeated, reversion must be to the beginning of the last such
top-level parenthesized group, including its repetition count.

Reviewed By: sscalpone, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D84281