Cache ISymbol.ToDisplayString results

[ToddGrun]

Instead of adding another CWT, went ahead and merged the CWT that we already had for INamedTypeSymbol into one that I needed for ISymbol/IAssemblySymbol.

Looked at one trace from speedometer runs with/without this change and filtered to callstacks containing ToDisplayString.

CPU went from 188 ms -> 54 ms. Allocations went from 9 MB -> 300 KB.

Commit 3 test insertion: https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/670197

[DustinCampbell]

This is really promising! I have some meta suggestions, since you said this is still a work-in-progress.

I'd rather see a more prescriptive approach rather than adding a ToCachedDisplayString(SymbolDisplayFormat?) method that calling code has to remember to call. I think it'd also be good to hide SymbolDisplayFormats altogether Instead, I'd rather see...

1. Every caller of ToDisplayString(SymbolExtensions.FullNameTypeDisplayFormat) should be updated to call the GetFullName extension method. Calling into the cache should be encapsulated by GetFullName.
2. A new GetGloballyQualifiedFullName extension method should be added and every caller of ToDisplayString(GloballyQualifedFullNameTypeDisplayFormat) should be updated to call GetGloballyQualifiedFullName.
3. A new method to replace all calls to ToDisplayString() with no SymbolDisplayFormat. (GetSimpleDisplayString()? GetDefaultDisplayString()?
4. An analyzer that flags calls to ToDisplayString(...) with a message to use one of the caching APIs.

With all of that, it should be harder to introduce new code that doesn't cache.

Thoughts?

[DustinCampbell]

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