[flang] Clean out obsolete parsing code. Handle !cdir$ fixed and free in parser.

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@fb2ff367ec0609b7307771f927f22fd7bab5e180
Reviewed-on: https://github.com/flang-compiler/f18/pull/34
Tree-same-pre-rewrite: false