[CursorInfo] Add a synthesized field

Mark implicit declarations with a `synthesized` field, since while we
still want them to have a cursor info result (eg. for their USR to find
possible overrides), it's likely that clients will want to treat them
differently to results that have a real location in source.

An alternative could be to remove the location entirely, but:
  - the module name would also have to be removed to prevent wasted
    lookups in the generated interface
  - having the location could still be useful as the location that
    caused the synthesized declaration (though at the moment only
    synthesized initializers have a location)

Resolves rdar://84990655

utils/gyb_sourcekit_support/UIDs.py

@@ -202,6 +202,7 @@ UID_KEYS = [
     # Before executing the actual request wait x ms. The request can be canceled
     # in this time. For cancellation testing purposes.
     KEY('SimulateLongRequest', 'key.simulate_long_request'),
+    KEY('IsSynthesized', 'key.is_synthesized'),
 ]