[flang] Check that various variables referenced in I/O statements may be defined

A number of I/O syntax rules involve variables that will be written to,
and must therefore be definable.  This includes internal file variables,
IOSTAT= and IOMSG= specifiers, most INQUIRE statement specifiers, a few
other specifiers, and input variables.  This patch checks for
these violations, and implements several additional I/O TODO constraint
checks.

Differential Revision: https://reviews.llvm.org/D86557