Adds AnnotationRegion, AnnotationSegment and AnnotatedAtom with
the annotation region table: a printer marks what each comment is,
the role of each of its parts and how comments nest, and a reader
gets the structure back from separatingAnnotationRegions() without
parsing text. The pieces of an AnnotatedAtom merge back, so the
separated string equals the print without marks, span for span.
MachOSwiftSection's and MachOObjCSection's marksCommentStructure
build on it (docs/AnnotationRegions.md).