Overlay.qll: discard predicates

for dbscheme elements with direct or indirect location links in dbscheme.

- Unify discardable entities under one Discardable superclass.
- Two discard predicates depending on TRAP ID type.
- Future-proof the XML and Yaml discard predicates for when their
  extractors become incremental.

python/ql/lib/python.qll

@@ -37,6 +37,7 @@ import semmle.python.pointsto.CallGraph
 import semmle.python.objects.ObjectAPI
 import semmle.python.Unit
 import site
+private import semmle.python.Overlay
 // Removing this import perturbs the compilation process enough that the points-to analysis gets
 // compiled -- and cached -- differently depending on whether the data flow library is imported. By
 // importing it privately here, we ensure that the points-to analysis is compiled the same way.