Work on scanning compiler directive lines.
Fix glitch in earlier change to parse-state.h.
Add ClassifyLine(), simplify some token sequence data lifetimes and interfaces.
Handle continued directives. Obey !DIR$ FIXED and FREE in prescanner.
Some refactoring of TokenSequence API, then support initial directives (FIXED, FREE, IGNORE_TKR).
Fix !DIR$ IGNORE_TKR syntax, manual was wrong.
Debugging directive scanning & parsing.
Profiling-directed speed-up - do not map cooked source locations to Provenance until a Message is emitted. Turn some non-nullable pointers into references.
Debugging.
Handle !DIR$ IVDEP too, it is in a test.
Accept directives in the execution part.
Original-commit: flang-compiler/f18@fb2ff367ec
Reviewed-on: https://github.com/flang-compiler/f18/pull/34
Tree-same-pre-rewrite: false