[flang] Refactoring: add SemanticsContext class

The new SemanticsContext holds the state of semantics whose lifetime
spans all of the compilations. It contains the scope tree (and so all
symbols), the intrinsics table, messages, and the state of options that
affect semantics (default kinds, search directories, etc.)

Original-commit: flang-compiler/f18@f04ceab81f383fa51d76e643066be251d6433ef2
Reviewed-on: https://github.com/flang-compiler/f18/pull/215
Tree-same-pre-rewrite: false