[analyzer] AnalysisDeclContext: Refactor and documentation

Summary:
`ScopeContext` wanted to be a thing, but sadly it is dead code.

If you wish to continue the work in D19979, here was a tiny code which
could be reused, but that tiny and that dead, I felt that it is unneded.

Note: Other changes are truly uninteresting.

Reviewed By: NoQ

Differential Revision: https://reviews.llvm.org/D73519