[clang][driver] Set the input type to Fortran when reading from stdin

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