[LLC] Generated fields include _tokenCredential even when ctor is disabled

[chamons]

If clients disable the default ctors, you will run into this error:

C:\Users\chhamo\Programming\azure-sdk-for-net\sdk\purview\Azure.Analytics.Purview.Catalog\src\Generated\PurviewTypes.cs(22,42): error CS0414: The field 'PurviewTypes._tokenCredential' is assigned but its value is never used [C:\Users\chhamo\Programming\azure-sdk-for-net\sdk\purview\Azure.Analytics.Purview.Catalog\src\Azure.Analyti

[chamons]

I looked into this. Here's what I found:

• It is non-trivial for the code generator to detect that the ctors have been suppressed. Suppression happens in the roslyn layer post codegen, so we'd have to wire up some sort of modeling or way of checking what's going to be suppressed.
• The use case (purview) was suppressing to customizing the ctor to model operations grouped together. I don't expect this is a common use case.

The current work around is either:

• Suppress the warning (that gets promoted to an error).
• Manually set these fields to something (unused) in the ctor

I am going to move this to our backlog for consideration in the future, if it becomes painful for future us.