[flang] Delay parse tree rewriting for I/O UNIT=func()

When an I/O statement's UNIT= specifier is a variable that is a
function reference, parse tree rewriting may determine the wrong type
of the result because generic resolution has not yet been performed.
So move this bit of parse tree rewriting into I/O semantic
checking so that the right handling (integer -> external file unit
number, character pointer -> internal I/O) applies.

Differential Revision: https://reviews.llvm.org/D135210