[Statement checker] Remove redundant state.

Remove the `TheFunc` state, which is used only in a few places and can
be trivially recomputed.

include/swift/AST/AnyFunctionRef.h

@@ -53,6 +53,20 @@ public:
     }
   }
 
+  /// Construct an AnyFunctionRef from a decl context that might be
+  /// some sort of function.
+  static Optional<AnyFunctionRef> fromDeclContext(DeclContext *dc) {
+    if (auto fn = dyn_cast<AbstractFunctionDecl>(dc)) {
+      return AnyFunctionRef(fn);
+    }
+
+    if (auto ace = dyn_cast<AbstractClosureExpr>(dc)) {
+      return AnyFunctionRef(ace);
+    }
+
+    return None;
+  }
+
   CaptureInfo getCaptureInfo() const {
     if (auto *AFD = TheFunction.dyn_cast<AbstractFunctionDecl *>())
       return AFD->getCaptureInfo();