[flang] Fix edge case with Hollerith
To prevent mistokenization of CHARACTER*2HXY as a Hollerith
literal constant while allowing it in DATA A/2*2HXY/, there's
a little state that tracks whether a / has been seen earlier
in the same statement. But it was being reset on each line,
not statement, so Hollerith in a DATA statement continuation
line was incorrectly tokenized. Fixed.
Differential Revision: https://reviews.llvm.org/D85571