Cache Go IR write-target construction

Avoid repeatedly evaluating the CFG-to-AST write-target mapping. Cold ReflectedXss profiles on CockroachDB and AWS reduce MkLhs domain evaluation from seven runs to one, with lower aggregate IR predicate time. End-to-end timings remain noisy.

Validated with 220 Go control-flow/data-flow tests including consistency queries, and identical full BQRS results on both databases.

Author: owen-mc

go/ql/lib/semmle/go/controlflow/IR.qll

@@ -1072,6 +1072,7 @@ module IR {
   }
 
   /** A representation of the target of a write instruction. */
+  cached
   newtype TWriteTarget =
     /** A left-hand side of an assignment. */
     MkLhs(ControlFlow::Node write, Expr lhs) {