Use discarded use site info to avoid unnecessary expensive work

[Youssef1313]

Probably helps with #67926 ?

My analysis is that GetUseSiteInfo is showing as expensive

The scenario is IDE completion where the IDE calls LookupSymbols here:

roslyn/src/Workspaces/Core/Portable/Recommendations/AbstractRecommendationServiceRunner.cs

Line 393 in ef625f3

_context.SemanticModel.LookupSymbols(position, container, includeReducedExtensionMethods: true));

So this scenario should mostly be using discarded diagnostics. My hypothesis is that we create non-discarded use site info to later add it in a discarded diagnostic bag, so my thought is that when we use non-discarded use site info, we call GetUseSiteInfo unnecessarily in some cases. So this PR aims to reduce the GetUseSiteInfo calls happening here:

roslyn/src/Compilers/CSharp/Portable/Symbols/SymbolExtensions.cs

Line 110 in ef625f3

var info = symbol.GetUseSiteInfo();

by getting into this code path when appropriate:

roslyn/src/Compilers/CSharp/Portable/Symbols/SymbolExtensions.cs

Lines 104 to 108 in ef625f3

	if (!useSiteInfo.AccumulatesDiagnostics)
	{
	Debug.Assert(!useSiteInfo.AccumulatesDependencies);
	return;
	}

Assuming this change is reasonable and correct, I'd still like someone confirm whether this is the root cause of #67926 or not.

[Youssef1313]

There are assertion failures, the change might not be correct then :/