[flang][f18] Make -fdebug-dump-{symbols|parse-tree} imply -fsyntax-only

The following _action_ options are always used with `-fsyntax-only`
(also an _action_ option):
  * -fdebug-dump-symbols
  * -fdebug-dump-parse-tree
This patch makes the above options imply `-fsyntax-only`.

From the perspective of `f18` this change saves typing and is otherwise
a non-functional change. But it will simplify things in the new driver,
`flang-new`, in which only the last action option is taken into account
and executed. In other words, the following would only run
`-fsyntax-only`:
```
flang-new -fdebug-dump-symbols -fsyntax-only <input>
```
whereas this would only run `-fdebug-dump-symbols`:
```
flang-new -fsyntax-only -fdebug-dump-symbols <input>
```

Differential Revision: https://reviews.llvm.org/D96528