[flang] Detect and rewrite ambiguous READ(CVAR)[,item-list]

READ(CVAR)[,item-list] with a character variable CVAR
could be parsed as an unformatted READ from an internal
unit or as a formatted READ from the default external unit
with a needlessly parenthesized variable format.  We parse
it as the former, but Fortran doesn't have unformatted
internal I/O.

Differential revision: https://reviews.llvm.org/D90493