BridgeJS: Support throws and async for closures

[krodak]

Overview

Adds throws(JSException) and async (and async throws(JSException)) support to BridgeJS-bridged closures, in both directions: closures Swift receives from JavaScript, and closures Swift hands to JavaScript. Previously closures were limited to synchronous, non-throwing signatures (#537).

Async settlement reuses the _bjs_makePromise + per-type Promise_resolve_<mangled> / Promise_reject machinery introduced in #758, so every bridged return type is inherited for free, including the associated-value enums added in #764; no parallel mechanism is introduced.

1. Effect-aware closure mangling. Closure ABI symbol names encode effects with the Swift ABI operators (Ya for async, K for throws), so signatures that differ only by effects no longer collide.

2. Throwing closures (both directions). A JavaScript callback's thrown JSException propagates into Swift; a Swift closure's thrown JSException is routed back to JavaScript. Only throws(JSException) is supported; plain throws is diagnosed.

3. Async closures (both directions). A JS-to-Swift callback is awaited via _bjs_awaitPromise; a Swift-to-JS closure returns a Promise settled via _bjs_makePromise. TypeScript surfaces these as (args) => Promise<R>. Supported return types mirror async functions, including @JS struct, enums (associated-value included), and Optional/Array/Dictionary compositions; the remaining unsupported async return types (protocols, namespace enums) are diagnosed rather than miscompiled.

4. Lifetime. The closure box survives across suspension without an explicit pin: the extracted closure value, captured by the settling Task, keeps it alive even if JavaScript releases the closure mid-flight. Covered by release-race and concurrent-invocation tests.

Commit structure

The work is split so the parts can be reviewed (or dropped) independently:

Commit 1 is the feature itself, self-contained.

Commit 2 adds a build-time warning for async throwing closures handed to JavaScript (introducing a warning severity to the BridgeJS diagnostics). It was written when the swiftlang/swift#89320 miscompile made that signature lose thrown errors at runtime; with commit 3 in place the warning no longer fires for a real risk, so this commit can be dropped, or kept if the warning-severity infrastructure is useful on its own.

Commit 3 works around swiftlang/swift#89320 by boxing JSException's stored properties into a private final class, shrinking the struct to a single stored reference so it travels in the direct typed-error convention and the miscompiled indirect-error path is never taken. The public API is unchanged; the cost is one heap allocation per thrown exception, on the error path only. With it, the previously gated Swift-to-JS async throws reject assertions run unconditionally and pass, and the JSClosure.async reject path gains its previously missing test. Removal once swiftlang/swift#89715 ships is tracked in #767 and is a library-internal layout change, not a breaking API change. Original exploration: PassiveLogic#13. The codegen-level counterpart for zero-parameter async throwing exports landed in #760 (tracked for removal in #761).

Tests

Codegen snapshots and end-to-end runtime round-trips (both directions, throwing and async, resolve and reject, Void, @JS struct, associated-value enum returns, the release-race, and concurrent calls) pass.

[kateinoigakukun]

Thanks!