Fix issue reporting diagnostic in additional file when diagnostic produced by a source generator

[dibarbet]

Resolves microsoft/vscode-dotnettools#2173

IDE diagnostic analyzers specifically go through a special path when reporting semantic diagnostics (different to other analyzers). This path had an old, bogus assert to verify that if the semantic diagnostic was in a file with no syntax tree (e.g. additional file), that also the project the file was in had no compilation (assuming it was a typescript project which has neither).

However this assert now gets hit because

1. Source generator diagnostics are reported as semantic diagnostics via an IDE diagnostic analyzer (link).
2. The Razor source generator reports diagnostics in additional files (the .razor file)

This change removes the assert as it doesn't appear to be necessary or valid.

[jasonmalinowski]

@dibarbet I agree on your analysis the assert was suspect, given the caller absolutely knew the situation it was in...

[CyrusNajmabadi]

Also approved :)