[flang] Add optional portability warning for upcoming Fortran 202X/3 breaking change

The soon-to-be-published next revision of the ISO Fortran language standard
contains a couple of breaking changes to previous specifications that may cause
existing programs to silently change their behavior.

For the change that introduces automatic reallocation of deferred length
allocatable character scalar variables when they appear as the targets
of internal WRITE statements, as IOMSG=/ERRMSG= variables, as outputs
of INQUIRE specifiers, or as INTENT(OUT) arguments to intrinsic
procedures, this patch adds an optional portability warning.

Differential Revision: https://reviews.llvm.org/D154242