[SourceKit][SymbolGraph] Add a 'ParentContexts' field the CursorInfo response

When the SymbolGraph json is requested via (key.retrieve_symbol_graph: 1) this adds
a new field in the response that lists all the parent contexts of the symbol under
the cursor with their symbol graph kind and name, and their USR:

key.parent_contexts: [
    {
      key.kind: "swift.struct",
      key.name: "Parent",
      key.usr: "s:27cursor_symbol_graph_parents6ParentV"
    },
    ...
  ]
}

Resolves rdar://problem/73904365

utils/gyb_sourcekit_support/UIDs.py

@@ -160,6 +160,7 @@ UID_KEYS = [
     KEY('RetrieveRefactorActions', 'key.retrieve_refactor_actions'),
     KEY('SymbolGraph', 'key.symbol_graph'),
     KEY('RetrieveSymbolGraph', 'key.retrieve_symbol_graph'),
+    KEY('ParentContexts', 'key.parent_contexts'),
     KEY('ActionUID', 'key.actionuid'),
     KEY('ActionUnavailableReason', 'key.actionunavailablereason'),
     KEY('CompileID', 'key.compileid'),