Use IIFE to capture variable for transformed dynamic import

[rbuckton]

In #49663 we fixed one issue related to closures and dynamic import, but introduced another. This changes the emit to use an IIFE to ensure the value of the variable is captured properly. We could potentially drop back to the _a = x, Promise.resolve().then(...) emit if we're not in any loop, but that would require tracking iteration statements as we descend, and dynamic import isn't likely to be used in a fast path that cares about the overhead of an IIFE.

Fixes #51554.

[Josh-Cena]

@rbuckton What do you think about the fix proposed in #51562? Frankly, I don't see the point to emit another closure just so that the temp variable works—directly coercing it to string in the current scope seems like more sensible behavior.

[rbuckton]

Closed in favor of #51562