[flang] Don't emit debugging output to module file

When a constant array value has a non-default lower bound, the current
expression formatting code uses a non-Fortran syntax to dump the lower
bounds. (There's no way in Fortran to explicitly specify such a constant value,
but they can be created through the use of named constants.)  But we
don't want this lower bounds syntax from expression dumping to show up
in module files, since it can't be parsed back in.  So disable that
part of expression formatting by default.

Fixes https://github.com/llvm/llvm-project/issues/64391.

Differential Revision: https://reviews.llvm.org/D157330