[flang][driver] Add extension options and -finput-charset

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

Differential Revision: https://reviews.llvm.org/D96407