[flang] Add AsFortran() for types

Add an `AsFortran()` member function to `DeclTypeSpec` and related
classes to convert types to a Fortran representation. This allows
error messages to mention types. It also fixes `.mod` file writing
where it previous used `operator<<` to convert types to Fortran.

Original-commit: flang-compiler/f18@eb16f8b5dd1b8c3631690afce03c3b26b26d0f6f
Reviewed-on: https://github.com/flang-compiler/f18/pull/521
Tree-same-pre-rewrite: false