CQ. Fix scoping of for-loop variables in initializers

Ensure loop-local variables introduced by `for` parts are bound
consistently during resolution, while still reporting reads that occur
before the declaration point.

The resolver now predeclares `ForPartsWithDeclarations` variables in the
loop’s local scope before traversing the initializer/condition/updaters.
This makes lexical lookup within `forLoopParts` resolve to the
loop-local element (even in `var x = x`), and allows later verification
to correctly report `REFERENCED_BEFORE_DECLARATION` instead of treating
the name as outer/undefined.

Also align foreach resolution by defining the declared loop variable
only after visiting the iterable expression, and update error
verification to hide not-yet-declared elements consistently for blocks,
switch members, and `for` parts via a generalized
`HiddenElements.forElements` helper.

Change-Id: Ifab327737bb033fe05f1d765196796924378ac5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/480120
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

tests/language/loop/for_test.dart

@@ -88,5 +88,6 @@ void testShadowLocal() {
   List<int> x = [1, 2, 3];
   for (var x = x; ;) break;
   //           ^
+  // [analyzer] COMPILE_TIME_ERROR.REFERENCED_BEFORE_DECLARATION
   // [cfe] Local variable 'x' can't be referenced before it is declared.
 }