Identical @using statements from imports or defaults override those from main document

[NTaylorMullen]

This results in 0 IntelliSense or coloring for using statements that match ones that exist on the main document.

The reason this occurs is we add using statement IR nodes as we encounter them and then make sure not to duplicate any. Problem is, we add ViewImports/default usings first which don't get source locations; when we encounter a using statement on the main document the using has already been added so we no-op in an effort to not duplicate an already added using.

[NTaylorMullen]

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