[flang][driver] Fix reading from stdin when using `-test-io`

This patch adds logic in the InputOutputTestAction frontend action for
reading input from stdin. Without this patch the following fails:
```
    flang-new -fc1 -test-io -
```

The implementation of `InputOutputTestAction` is cleaned-up and a test
for reading from stdin is added.

Note that there's a difference between `-test-io` and e.g. `-E` in terms
of file I/O. The frontend action for the former handles all file I/O on
it's own. Conversely, the action corresponding to -E relies on the
prescanner API to handle this.

Currently we can't test reading from stdin for `flang-new -`. In this
case `libclangDriver` assumes `-x -c`. This in turn leads to `flang-new
-cc1`, which is not supported.