[flang] Make the frontend driver error out when requesting multiple actions

With this change, the following invocations will be treated as errors
(multiple actions are specified):
```
$ flang-new -fc1 -E -fsyntax-only file.95
$ flang-new -fc1 -fsyntax-only -fdebug-dump-symbols file.95
```
In the examples above it is not clear whether it is `-fsyntax-only` or
the other action that is run (i.e. `-E` or `-fdebug-dump-symbols`). It
makes sense to disallow such usage. This should also lead to cleaner and
clearer tests (the `RUN` lines using `%flang_fc1` will only allow one
action).

This change means that `flang-new -fc1` and `clang -cc1` will behave
differently when multiple action options are specified. As frontend
drivers are mostly used by compiler developers, this shouldn't affect or
confuse the compiler end-users. Also, `flang-new` and `clang` remain
consistent.

Tests are updated accordingly. More specifically, I've made sure that
every test specifies only one action. I've also taken the opportunity to
...