[flang] Move features.h from parser to common

Which features are enabled and disabled applies to more than just the
parser, so move that functionality to `common`.

Original-commit: flang-compiler/f18@98b3240efc25095b6bdfa43280513f2afcac99e1
Reviewed-on: https://github.com/flang-compiler/f18/pull/815
Tree-same-pre-rewrite: false