LSP: Handle multiple project context for the same file

[0101]

Deal with a situation where a file is in multiple different projects that can be active at the same time, or when a project has multiple targets.

Will probably need a different solution for VS and public LSP.

[0101]

VS is already sending this info, we just need to deserialize part of the message to VSInternalTextDocumentIdentifier.

[abonie]

Could be related to why diagnostics currently are missing project info #17973