Box JSException storage in a class to fit the direct typed-error convention

[krodak]

Overview

An intermediate solution for the async closure reject path broken by swiftlang/swift#89320, until the IRGen fix in swiftlang/swift#89715 ships.

JSException (~36 bytes) exceeds the direct typed-error convention on wasm32, so an async throws(JSException) closure value takes the indirect-error path that swiftlang/swift#89320 miscompiles: when the closure is captureless, the thrown error is corrupted across the async unwind and the rejected Promise receives garbage instead of the JSException. Boxing JSException's storage into a single class reference (~4 bytes) moves it into the direct convention, so the broken path is never taken.

The function-export variant of the same bug was fixed separately at the codegen level in swiftwasm#760 (already on main); this PR addresses the closure side, which codegen cannot reach (the closure value is constructed by user code).

1. Boxing, conditional on compiler(>=6.3). thrownValue, description, and stack move into a private final class; the struct holds one stored reference. The public surface is unchanged (computed accessors, same initializers, equivalent Equatable); the cost is one heap allocation per thrown exception, on the error path only. The boxed layout crashes the Swift 6.1 wasm IRGen (Invalid InsertValueInst operands), so older compilers keep the original stored properties and today's behavior. 6.2 is unverified and conservatively kept on the legacy layout; the threshold can be lowered after verification.

2. Reject tests gated by capability, not blanket. A @JS probe (asyncThrowsClosureRejectSupported) reports whether the boxed layout is active; the Swift-to-JS async throws(JSException) closure reject assertions run whenever it is. On Swift 6.3 and later they run and pass; on 6.1 they are skipped (status quo there). The blanket ASYNC_THROWS_CLOSURE_REJECT_BLOCKED gate from the base PR is removed.

3. Base-PR mitigations removed. With boxing active on current toolchains, the build-time warning and most doc callouts from the base PR are dropped; the support table notes the reject path requires Swift 6.3 or later.

4. JSClosure.async reject path. The same bug shape existed in JSClosure.async and was untested; this PR adds the missing reject-path test (gated on compiler(>=6.3)), which passes with boxing.

Notes

• Stacked on the closures feature PR (BridgeJS: Support throws and async for closures #11) so the diff shows only the boxing change and its consequences.
• The underlying compiler bug remains open; this is a layout-level mitigation that stays harmless (arguably beneficial, as it shrinks a frequently-copied error type) after IRGen: fix async typed throws miscompiles on Wasm swiftlang/swift#89715 lands.
• Tests pass end to end on Swift 6.3 (including the formerly gated closure reject cases) and the legacy path builds and passes as before on 6.1.