Add support for the following layout options:
The default fixed line length in FrontendOptions is 72, based off the current default in Fortran::parser::Options. The line length cannot be set to a negative integer, or a positive integer less than 7 excluding 0, consistent with the behaviour of gfortran.
This patch does not add `-ffree-line-length-n` as Fortran::parser::Options does not have a variable for free form columns.
Whilst `fixedFormColumns` variable is used in f18 for `-ffree-line-length-n`, f18 only allows `-ffree-line-length-none`/`-ffree-line-length-0` and not a user-specified value. `fixedFormcolumns` cannot be used in the new driver as it is ignored in the frontend when dealing with free form files.
Options `-ffree-form` and `-ffixed-form` do not have help text as the options are defined with a `BooleanFFlag` type. This introduces `-fno-free-form` and `-fno-fixed-form` which is not used by the new driver and only exists for gfortran. Help text is therefore duplicated between `-f` and `-fno` which does not belong in `flang-new --help`.
Summary of changes:
- Create AddFortranDialectOptions method in Flang.cpp
- Create FortranForm enum in FrontendOptions.h
- Add fortranForm_ and fixedFormColumns_ to Fortran::frontend::FrontendOptions