[flang][driver] Add forced form flags and -ffixed-line-length

Add support for the following layout options:
* -ffree-form
* -ffixed-form
- -ffixed-line-length=n (alias -ffixed-line-length-n)
Additionally remove options `-fno-free-form` and `-fno-fixed-form` as they were initially added to forward to gfortran but gfortran does not support these flags.

This patch adds the flag FlangOnlyOption to the existing options `-ffixed-form`, `-ffree-form` and `-ffree-line-length-` in Options.td. As of commit 6a754968, these flags are not currently forwarded to gfortran anyway.

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 the `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.

Summary of changes:
- Remove -fno-fixed-form and -fno-free-form from Options.td
- Make -ffixed-form, -ffree-form and -ffree-line-length-n FlangOnlyOption in Options.td
- Create AddFortranDialectOptions method in Flang.cpp
- Create FortranForm enum in FrontendOptions.h
- Add fortranForm_ and fixedFormColumns_ to Fortran::frontend::FrontendOptions
- Update fixed-form-test.f so that it guarantees that it fails when forced as a free form file to better facilitate testing.

Differential Revision: https://reviews.llvm.org/D95460