Port TypeScript PR #62661: fix [Symbol.iterator]() lost on union with never

[Copilot]

Ports microsoft/TypeScript#62661.

getIterationTypesOfIterable wasn't calling getReducedType, so non-iterable types reducible to never (e.g. { t: "a" } & { t: "b" }) weren't being erased from unions before iteration checking. This caused [Symbol.iterator]() to be "lost":

declare var x: number[] | ({ t: "a" } & { t: "b" });
let [el2] = x; // incorrectly errored with TS2488
for (const elem of x) {} // incorrectly errored with TS2488

• Add t = c.getReducedType(t) at the top of getIterationTypesOfIterable in internal/checker/checker.go
• Baseline diff for iterableWithNeverAsUnionMember deleted — output now matches upstream

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[Copilot]

Pull request overview

This PR ports TypeScript PR #62661, which fixes a bug where [Symbol.iterator]() was incorrectly reported as missing on union types containing never-reducible intersections (e.g., number[] | ({ t: "a" } & { t: "b" })). The fix adds a single line to reduce types before iteration checking, ensuring that impossible intersections are eliminated from unions before checking for iterator methods.

Changes:

• Add getReducedType call in getIterationTypesOfIterable to reduce types before iteration checking
• Eliminate baseline diff file for iterableWithNeverAsUnionMember test, indicating convergence with upstream TypeScript behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/checker/checker.go	Added t = c.getReducedType(t) at the start of getIterationTypesOfIterable to reduce never-intersections before iteration checking
testdata/baselines/reference/submodule/compiler/iterableWithNeverAsUnionMember(target=esnext).errors.txt.diff	Removed diff file (now empty), indicating the port now matches upstream behavior
testdata/baselines/reference/submodule/compiler/iterableWithNeverAsUnionMember(target=esnext).errors.txt	Updated baseline showing 3 errors instead of 6 (3 spurious errors eliminated)