[gfortran PR32147] Make module files more predictable

[Tobias Schlüter]

The PR is concerned with the issue that our module files change more or 
less randomly if one changes the sources only slightly.  This is because 
we build up our symbol trees balancing them randomly, and then traverse 
them top-down.

This patch changes the order to always be left-to-right.  This is fairly 
trivial, and amounts to replacing
{
    walk_left ();
    walk_right ();
    do_something ();
}
with
{
    walk_left ();
    do_something ();
    walk_right ();
}
keeping in mind that do_something() may return early, which made a few 
safeguards necessary.

Unfortunately the bug reporter didn't have time to verify that it indeed 
fixes his problem, but due to the trivial nature of this patch, I think 
it's worthy having anyway.  It also gives us cleaner -fdump-parse-tree 
output after all! :-)

Built and tested on i386-darwin.  Ok?

Cheers,
- Tobi
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