[flang] Fix "non-advancing" I/O, support $ in FORMAT

[flang] Fix "non-advancing" I/O, support $ in FORMAT

Non-advancing I/O was failing; ExternalFileUnit was losing
track of what writes had been committed to the file.  Fixed.
Also, support the common extension of $ and \ in a FORMAT
as being equivalent to ADVANCE=NO.

Differential Revision: https://reviews.llvm.org/D105046