[flang][driver] Add support for `-I` in the new driver

Add support for option -I in the new Flang driver. This will allow for
included headers and module files in other directories, as the default
search path is currently the working folder. The behaviour of this is
consistent with the current f18 driver, where the current folder (i.e.
".") has the highest priority followed by the order of '-I's taking
priority from first to last.

Summary of changes:
- Add SearchDirectoriesFromDashI to PreprocessorOptions, to be forwarded
  into the parser's searchDirectories
- Add header files and non-functional module files to be used in
  regression tests. The module files are just text files and are used to
  demonstrated that paths specified with `-I` are taken into account when
  searching for .mod files.

Differential Revision: https://reviews.llvm.org/D93453