[analyzer] Refactor doc comment processing like a state machine

As suggested in https://dart-review.googlesource.com/c/sdk/+/317446.

This introduces a new class CharacterSequence, an abstraction over the
two different doc comment types, and the two remarkably different ways
they need to be walked.

One good example of the complexities is that a line starting with
`// ` must be completely ignored when processing a single-line doc
comment, but must not be ignored when processing the lines of a
multi-line doc comment. Implementing the logic of the two styles in
one body was too ugly and complex.

This also fixes a bug in offsets, as can be seen in the
ast_builder_test.

Work towards #50702

Change-Id: Ife5f17a88268e5cda1ac89d4c14fa1f1cad1333f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321360
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>