Support "View Source" link for assembly sourced metadata

[JeremyCaney]

Currently, the "View Source" link is only displayed for API documentation is the API's metadata is sourced from a project file (e.g., *.csproj). If the metadata is instead sourced directly from code (e.g., *.cs) or a compiled assembly (i.e., *.dll), then the "View Source" (and the corresponding "Improve this Doc") link is not presented.

I don't believe a standard assembly (i.e., *.dll) contains this information. Thus, in order to be technically feasible, I assume this would necessitate the presence of a corresponding symbols file (i.e., *.pdb). In addition, it may be useful to evaluate the JSON mappings provided by Source Link—assuming they're available in the symbols file—as this would provide support for assemblies compiled from separate repositories. Otherwise, the *.pdb data is only useful if the references maps to files in the local repository.

That said, I'm not personally familiar enough with the APIs available for symbol files to know how involved this is.