Improve conflict marker detection and fixing

Merge release/dev16.6-preview2 to master

Vary async iterator test names

Dismiss Completion when Signature Help is explicitly invoked

🧹 Remove unnecessary suppressions

This will help us identify which of the tests is the cause of #42590

Apply color scheme when in high contrast

* Separate top-level and nested nullability checks in OHI

* Update test

* Remove mention of top-level in message

* DeclareAsNullable on invocation with null

* Fix from assignment

* Use different equivalence key for null argument

* Recognize null assignments to fields and properties

* Address feedback from Cyrus

* Annotate nullability

If we fail to create the sqlite db, don't continually retry for every client that wants it.

Be more resilient to exceptions happening in sqlconnection.

Infer better variable names for a foreach expression.

Remove hidden extract method option.

Move UseCoalesceExpression analyzers/fixers/tests to shared layer

Co-Authored-By: NSam Harwell <sam@tunnelvisionlabs.com>