Call Hierarchy

[svenefftinge]

The LSP should define a request from the client to server to obtain callers for a definition as well as obtaining the containing callable definitions (e.g. containing method declaration).
Based on that an editor can provide a call hierarchy helping the user to understand the code better.

[MarkZ3]

Cquery has an extension for this. Perhaps a god place to start:
https://github.com/cquery-project/cquery/blob/master/src/messages/cquery_call_hierarchy.cc#L23

[dbaeumer]

See also the discussion here microsoft/vscode-languageserver-node#346 about and idea to how more general implement this.