[flang] More compile-time error checking for null implied DO loops in array constructors

An implied DO loop in an array constructor may not have a type (explicit
or otherwise) with a character length that depends on a value of an
implied DO index or a non-constant expression if the implied DO loop
executes no iterations.  When the iteration count can be known to be
zero at compilation time, catch the case of a non-constant length
expression correctly.

Differential Revision: https://reviews.llvm.org/D156753