[flang][runtime] Don't skip input spaces when they are significant

When formatted input (not list-directed or NAMELIST) is in "BZ" mode,
either because a BZ control edit descriptor appeared in a FORMAT or
BLANK="ZERO" appeared in OPEN or READ, input editing must not skip
over blanks before or within the input field.

Differential Revision: https://reviews.llvm.org/D123725