[Flang][Driver] Add PrintPreprocessedInput FrontendAction (`flang-new -E`)

This patch implements the first frontend action for the Flang parser (i.e.
Fortran::parser). This action runs the preprocessor and is invoked with the
`-E` flag. (i.e. `flang-new -E <input-file>). The generated output is printed
to either stdout or the output file (specified with `-` or `-o <output-file>`).

Note that currently there is no mechanism to map options for the
frontend driver (i.e. Fortran::frontend::FrontendOptions) to options for
the parser (i.e. Fortran::parser::Options). Instead,
Frotran::parser::options are hard-coded to:

```
std::vector<std::string> searchDirectories{"."s};
searchDirectories = searchDirectories;
isFixedForm = false;
_encoding(Fortran::parser::Encoding::UTF_8);
```

These default settings are compatible with the current Flang driver. Further
work is required in order for CompilerInvocation to read and map
clang::driver::options to Fortran::parser::options.

Co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>

Differential Revision: https://reviews.llvm.org/D88381