[analzyer] Refactor analyzeAnalysisOptions into a class.

Replaces the top level function `analyzeAnalysisOptions`, which
contained 3 nested local functions, with a class
`AnalyzeAnalysisOptions`, where the nested local functions have been
changed into private methods.

All the variables in the top level function that were implicitly
write- or read-captured by the nested local functions have been
changed into class fields or explicit method parameters. This should
make the code a lot easier to follow.

In this CL, I've tried hard to preserve the existing functionality
exactly; besides converting local variables to fields and parameters,
I haven't really made any changes to the core logic beyond code
motion. In follow-up CLs, I plan to make some simplifications that are
less obviously semantics preserving (but should be easier to review
since they won't involve so much code motion). Once that is done I
hope to be able to migrate the error reporting in this class to use
the literate diagnostic API.

Change-Id: I6a6a696414ecae80535c8d6633eaa567d37dcccc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/479080
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>