Add the following options:
- -fimplicit-none and -fno-implicit-none
- -fbackslash and -fno-backslash
- -flogical-abbreviations and -fno-logical-abbreviations
- -fxor-operator and -fno-xor-operator
- -falternative-parameter-statement
- -finput-charset=<value>
Summary of changes:
- Enable extensions in CompilerInvocation#ParseFrontendArgs
- Add encoding_ to Fortran::frontend::FrontendOptions
- Add encoding to Fortran::parser::Options
Depends on: D96344
[nit] ALWAYS is a bit unclear to me. Perhaps WITH_IMPL_NONE?