Extensions: fix RemoveUnusedMember scenarios

[jcouv]

Fixes #80645 and related issues #81213 and #81657

The principle of AbstractRemoveUnusedMembersDiagnosticAnalyzer is that we respond to member declarations by registering them (AnalyzeSymbolDeclaration), then analyze various references (invocations, member references, deconstructions, nameof, cref, etc), then when we exit the type we can report private unused members.

The change is that we treat extension members as part of the enclosing static class:

• we don't report unused private members until we exit the enclosing static class
• we enumerate the extension members along with members of the enclosing static class

Additionally, any usage of disambiguation syntax marks the corresponding extension member as "used".

[CyrusNajmabadi]

Suggested change

	{
	symbolStartContext.RegisterSymbolEndAction(symbolEndContext => OnSymbolEnd(symbolEndContext, hasUnsupportedOperation));
	}
	symbolStartContext.RegisterSymbolEndAction(symbolEndContext => OnSymbolEnd(symbolEndContext, hasUnsupportedOperation));
	``` #ByDesign

[jcouv]

I kept those braces as neither the condition nor the branch are simple, so find this this easier to read

[CyrusNajmabadi]

Suggested change

	{
	return false;
	}
	return false;

Also, consider a comment above the if. but ok if not present. #Resolved

[CyrusNajmabadi]

Suggested change

	{
	yield return extensionMember;
	}
	yield return extensionMember;
	``` #Resolved

[CyrusNajmabadi]

Suggested change

	if (extensionBlockMethod.AssociatedSymbol is { } associatedSymbol)
	{
	AddIfCandidateSymbol(builder, associatedSymbol);
	}
	AddIfCandidateSymbol(builder, extensionBlockMethod.AssociatedSymbol);
	``` #Resolved

[CyrusNajmabadi]

Suggested change

	foreach (var extensionMember in extensionBlock.Members)
	{
	yield return extensionMember;
	}
	foreach (var extensionMember in extensionBlock.Members)
	yield return extensionMember;
	``` #Resolved

[CyrusNajmabadi]

you can just inline 'code' and remove the 'fixedcode=code' line in all of these. then make the methods not async/awit. but instead just return the new Test { ... }.RunAsync(); #Resolved

[CyrusNajmabadi]

Suggested change

	if (associated is null)
	continue;
	``` #Resolved

[jcouv]

Sorry, I'd misunderstood the suggestion in previous PR because of how the diff is shown in CodeFlow...