Python: cache shared CFG scope

Cache the Python-local shared-CFG scope relation after semantic inputs and before staged SSA and dataflow consumers.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 529363f5-bc7d-4f0b-9f47-e03ba9aa0cdf

Author: yoff

python/ql/lib/semmle/python/controlflow/internal/Cfg.qll

@@ -68,6 +68,7 @@ class ControlFlowNode extends CfgImpl::ControlFlowNode {
   BasicBlock getBasicBlock() { result = super.getBasicBlock() }
 
   /** Gets the scope containing this flow node. */
+  cached
   Py::Scope getScope() { result = super.getEnclosingCallable().asScope() }
 
   /** Gets the enclosing module. */