Fixes #35203

1. Improve the message for unused value assignment diagnostic to make it clear that we are flagging an unnecessary assignment, not an unused variable
2. Add descriptions for unused value assignment and unused parameter diagnostics to allow users to specify special discard symbol names that are respected by the analyzer. There are various cases where user wants to retain unused parameters (signature compat, documentation) and unnecessary assignments to locals (ease of debugging), but do not want to blanket suppress these diagnostics.

* perf tweak to run OpenFileOnly analyzers only for open files even if full solution analysis is on but run for closed files for fix all.

also made add using test to excerice OOP code properly

* addressed PR feedbacks

* added more comments

* changed the way we report live analysis to task center

previously, we listen to diagnostic service to report progress. problem is that, it only raise event if it found errors on a file. so what we report is actually last file we found errors on rather than file that we are analyzing.

this caused confusion since we report in task center that we are analyzing file "A" when it is actually "analyzed" not "analyzing"

another issue is since it only report file that contains errors. we might not actually show anything in task center if there is no error, or show file "A" for long time if that is only file with errors.

this PR changes the experience closer to what users would expects. and now progress is for solution crawler not specifically on diagnostics.

now we report file that solution crawler is analyzing.

there is still cavet such as solution cralwer can pause between processing a file if VS is busy. but it will still show file "A". or we will not update UI at least 200ms a part and etc.

since it is task center where we don't want to be too impactful to VS, based on feeedback we will see whether we need to do more such as detect solution crawlwer pause and update task center to show pasue. or update task center to show different stage such as analyzing/analyzed.

or show in task center, what analyzer is actually running such as diagnostic, todo, designer attribute scan, find all reference cache and etc.

* addressing PR feedbacks

* Implement MetadataAsSource for ref and readonly structs

* MetadataAsSource for readonly members

* Fix NotImplementedException errors. Fix some ArrayBuilder leaks.

* Fix a leak. Fix implicit readonly attribute test.

* List passed to GetUpdatedDeclarationAccessibilityModifiers needs to support Remove

* Allow specifying a metadata language version for MetadataAsSourceTests

* Remove unused SyntaxTokenList members. Comment readonly event generation.

* Check VB MetadataAsSource for readonly members

* Fixes from feedback

* Add test for not implicitly readonly struct getter

* Label arguments in call to GetCustomAttributesForToken

* Reference issue about readonly event public API

Don't add null checks for nullable-structs.  The fact that they're nullable indicates they don't need checks.

* Add import placement codestyle option

* Add TextEditor CodeStyle options for Using Placement

* Add misplaced using directives analyzer and code fix

* Use consistent pluralization

* Removed Preserve AddImportPlacement option

* Removed Preserve from CSharp Style Options

* Removed Preserve from editorconfig tests

* Coverted to Roslyn style analyzer and fixer tests.

* Simplified MisplacedUsings CodeFix based on feedback.

* Simplified MisplacedUsings CodeFix based on feedback.

* Add warning annotation to moved using directives

* Move misplaced usings out of multiple namespaces

* Deduplicate usings when moving them

* Simplified move usings warning text

* Add expected warning markers to misplaced using tests

* Fix editor config generator tests

* Consolidated diagnostics and tests

* Add tests where directives are in both contexts