In clients that support DiagnosticRelatedInformation, display
clang notes as these nested diagnostics rather than appending
them to the parent diagnostic's message. Behaviour for clients
that don't support related information should be unchanged.
This fixes a plethora of "not indexed" errors when the document has not been indexed.
* Message handler throws NotIndexed if not overdue
* The message is put into backlog and tagged with backlog_path
* path2backlog[path] tracks backlog associated with document `path`
* The backlog is cleared when the index is merged
* backlog[0] is forced to run if it becomes overdue
textDocument/{declaration,definition,typeDefinition} return either LocationLink[] or Location[]
Add an initialization option client.linkSupport . When it is false, ccls will return Location[] disregarding client's linkSupport.
`struct LocationLink` does not include originSelectionRange as it is wasteful.
