This patch makes sure that for the following invocation of the new Flang
driver, clangDriver sets the input type to Fortran:
```
flang-new -E -
```
This change does not affect `clang`, i.e. for the following invocation
the input type is set to C:
```
clang -E -
```
This change leverages the fact that for `flang-new` the driver is in
Flang mode.
Differential Revision: https://reviews.llvm.org/D96777
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.
Andrzej Warzynski