[flang] Use faster path for default formatted character input

Rather than reading default character variables in formatted
input one byte at a time via NextInField(), skip and read
them via blocks of available buffer data.  This eliminates
a bottleneck that affected reads of large character values.
(It also exposed a problem with sequential reads with RECL=
set on the OPEN statement, so that's fixed too.)

Differential Revision: https://reviews.llvm.org/D121144