Synchronize AnalyzerConfig documents when creating OOP solutions

Ensure we set _isSubscribedToSolutionEvents

Fixes #31685

* Add reproducer for #34872

* Fix bug and add test

* WIP

* Test similar scenario in VB

* Add void check in ClassifyConversionFromType, update tuple tests

* Cleanup

If we don't set this, we'll subscribe to solution events multiple times
which isn't great. Our event handler is idempotent so it'll still do
the right thing, but we'll just be wasting extra time processing
notifications that won't do anything.

Fixes https://github.com/dotnet/roslyn/issues/35372

* Don't emit readonly attributes on members within readonly structs

* Check ContainingType.IsReadOnly before EnsureIsReadOnlyAttributeExists just for consistency

Improve diagnostic messaging for unused value assignment and unused p…

* Symbol display for readonly members

* Add coverage

* Test malformed properties and events

* Show readonly keyword on non-field members in QuickInfo

* Use IsDeclaredReadOnly for symbol display (WIP)

* Update based on design and fix some test failures

* Add readonly keywords to various IL tests

* Fix more tests, respond to feedback

* Fix compile error

* Don't display readonly keyword for implicit readonly getters from source

* Don't show readonly keyword for members contained within a readonly struct

* Move from extension methods to private statics

Merge release/dev16.1 to master

* Nullable Deconstruction in foreach loops:
- Visit deconstruction in foreach loops, rather than handling it directly
- Add tests

Implements a design change where overrides are allowed to change the
type of the member as long as there is an implicit nullable reference
conversion from the overriding type to the overridden type according to
the nullable variance rules.

Fixes #23268
Fixes #30958

It's not as important as the others because the file ordering doesn't
matter in the same way, but it's good to have for consistency.