[flang] Fix repeated "DT" editing

User-defined derived type editing in formatted I/O wasn't
working with repeat counts; e.g., "2DT(10)".  The solution required
some code to be moved from GetNextDataEdit() to CueUpNextDataEdit() so
that a stack entry for a nonparenthesized repeated data edit
descriptor would work correctly -- all other data edit descriptors
are capable of dealing with repetition in their callees, so the bug
hadn't been exposed before.

Debugging this problem led to some improvements in error messages
for bad format strings, and those changes have been retained; also,
a dead member function was discovered and expunged.

Differential Revision: https://reviews.llvm.org/D117904