Add support for jump to definition for stdlib/Foundation symbols

[adam-fowler]

This work involves generating an interface view for a module.
Working out the location of the symbol within this generated file.
Somehow passing all of this in a response.

How this is all done and how much of it should be up to sourcekit-lsp is undefined at this point in time. I've basically added this as a placeholder for work to be done in the future, once we have a better idea what that work should be.

[adam-fowler]

I see SourceKit-LSP already supplies the path to the .swiftinterface file in the textDocument\definition response. If SourceKit-LSP could provide an actual position in that file and we had a method of combining the .swiftinterface and the related .swiftdoc we could possibly do a lot of the work for this on the VS Code side.

[ahoppen]

rdar://99329577

[adam-fowler]

See swiftlang/vscode-swift#408 for update