[deps] Add DeclareLocal to ReactiveScope decls

--- 

Try to fix bug from #1589: 

> If a declaration for an immutable identifier (i.e. one that is not later 
re-assigned, since undefined is a primitive) is sandwiched between mutations, we 
currently do not record it as an output or hoist it out of the reactive scope. 

One simple fix is to add all declared (and later referenced) identifiers as 
declarations of a reactive scope. This has some undesired effects (e.g. 
additional instructions + memo cache slots), but in practice, this shouldn't be 
happening often. 

Alternatively, we could 1.) add a pass to hoist declarations, 2.) account for 
this in constant propagation, or 3.) add a bailout

compiler/forget/src/ReactiveScopes/PropagateScopeDependencies.ts

@@ -537,6 +537,15 @@ class PropagationVisitor extends ReactiveFunctionVisitor<Context> {
         id,
         scope: context.currentScope,
       });
+    } else if (value.kind === "DeclareLocal") {
+      // Some variables may be declared and never initialized. We need
+      // to retain (and hoist) these declarations if they are included
+      // in a reactive scope. One approach is to simply add all `DeclareLocal`s
+      // as scope declarations.
+      context.declare(value.lvalue.place.identifier, {
+        id,
+        scope: context.currentScope,
+      });
     } else if (value.kind === "Destructure") {
       context.visitOperand(value.value);
       for (const place of eachPatternOperand(value.lvalue.pattern)) {

compiler/forget/src/__tests__/fixtures/compiler/uninitialized-declaration-in-reactive-scope.expect.md

@@ -16,22 +16,25 @@ function Component(props) {
 ```javascript
 import { unstable_useMemoCache as useMemoCache } from "react";
 function Component(props) {
-  const $ = useMemoCache(2);
+  const $ = useMemoCache(3);
+  let y;
   let x;
   if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
     x = mutate();
-    let y;
+
     foo(x);
-    $[0] = x;
+    $[0] = y;
+    $[1] = x;
   } else {
-    x = $[0];
+    y = $[0];
+    x = $[1];
   }
   let t0;
-  if ($[1] === Symbol.for("react.memo_cache_sentinel")) {
+  if ($[2] === Symbol.for("react.memo_cache_sentinel")) {
     t0 = [y, x];
-    $[1] = t0;
+    $[2] = t0;
   } else {
-    t0 = $[1];
+    t0 = $[2];
   }
   return t0;
 }