[flang] Fix non-deterministic line output function

The evaluation order for the `|` operator is undefined
(in contrast to the short-circuiting `||` operator). The arguments are
stored in variables to force a specific evaluation order.

A test in D107575 relies on this change.

Reviewed By: kiranchandramohan, klausler

Differential Revision: https://reviews.llvm.org/D108623