[flang] Fix source line continuation in potential macro calls (bugzilla 46768)

The prescanner looks for implicit continuation lines when
there are unclosed parentheses at the end of a line, so that
source preprocessing macro references with arguments that span
lines are recognized.  The condition that determines this
implicit continuation has been put into a predicate member
function and corrected to apply only when the following line
is source (not a preprocessing directive, comment, &c.).

Fixes bugzilla #46768.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D84280