Add duplicate identifier error for using declarations shadowed in for-of loops

[Copilot]

When a using declaration in a for-of loop head has the same name as a declaration in the loop body, the transformed output generates conflicting variable declarations that can cause runtime errors. This PR adds a compile-time error to catch this issue early.

Problem:

class Foo {}
for (using foo of [{ [Symbol.dispose]() {} }]) {
  const foo = new Foo(); // This shadows the using declaration
}

When compiled to ES5, this generates:

// ... transform helpers ...
try {
    var foo = __addDisposableResource(env_1, foo_1, false);
    var foo = new Foo(); // Redeclaration error at runtime
}

Solution:
Added a new check in checkForOfStatement that detects when:

• A variable declaration list has the Using flag (covers both using and await using)
• The target language version requires downleveling (< LanguageFeatureMinimumTarget.UsingAndAwaitUsing)
• The loop body contains variable declarations with the same names as the using declarations

When these conditions are met, TypeScript now reports:

error TS2300: Duplicate identifier 'foo'.

Key Features:

• Only applies when downleveling is required (no error in ESNext target)
• Handles both using and await using declarations
• Recursively searches all statements in the loop body, including nested blocks
• Works with all variable declaration types (const, let, var)

Testing:
Added comprehensive test cases covering various scenarios and verified that existing functionality remains unaffected.

Fixes #61714.

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[RyanCavanaugh]

You should not leave TODOs in your code. You didn't need two testcases here since one already tests both configurations. There was no need to typecast both .escapedTexts to string.

Start over from scratch. Duplicate identifier logic generally occurs in the binder. Look at how other cases work.

[Copilot]

You should not leave TODOs in your code. You didn't need two testcases here since one already tests both configurations. There was no need to typecast both .escapedTexts to string.

Start over fro...

I've completely rewritten the implementation based on your feedback. The duplicate identifier logic now correctly resides in the binder (rather than the checker), following the same pattern as other duplicate identifier cases. I've also simplified the test case and removed unnecessary type assertions and TODOs. The fix is now implemented in bindForInOrForOfStatement and properly detects shadowing only when using declarations will be downleveled. Commit f6e16c3.

Warning

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If you need me to access, download, or install something from one of these locations, you can either:

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[RyanCavanaugh]

This seems worse than a proper fix, my bad.