[flang] Simplify unparse-with-symbols.cc

Change it so that the first time a symbol is referenced, its full
definition is emitted, prefixed by "DEF:", and subsequent references
are prefixed with "REF:".

This doesn't give exactly the same results as before but it is
sufficient for testing and is simpler and easier to maintain.
The expected results need to be tweaked accordingly.

Original-commit: flang-compiler/f18@27b5d082a80f5e98b1e85a503518e7aa8c806af7
Reviewed-on: https://github.com/flang-compiler/f18/pull/214
Tree-same-pre-rewrite: false