BridgeJS: Support optional @JS struct in imported function signatures

[krodak]

Overview

Optional @JS structs could not be used as parameters or return values of imported (@JSFunction) signatures. While an optional primitive works, an optional @JS struct did not:

@JS struct Point { var x: Int; var y: Int }

@JSFunction func roundTrip(_ point: Point?) throws(JSException) -> Point? // did not compile

The code generator lowered Optional<Point> using the non-optional object-id ABI ([isSome, objectId] for parameters, a single Int32 word for returns), for which no Optional lowering exists in the runtime, so the generated thunk failed to compile.

This change bridges optional @JS structs through the stack ABI instead, an isSome discriminator plus the struct fields, exactly like optional arrays and dictionaries. @JS structs already conform to the stack-based bridging protocols, so the runtime's _BridgedAsOptional/stack extensions and the JS link's stack handling already support this shape. Only the import-side lowering and lifting in the code generator needed to change:

1. Parameter lowering. .nullable(.swiftStruct) in the import context now lowers to a single isSome flag, with the struct transferred on the stack, rather than appending the non-optional objectId word.

2. Return lifting. The optional struct return is now lifted from the stack rather than from a single object-id word.

Generated thunk, before:

let (pointIsSome, pointObjectId) = point.bridgeJSLowerParameter() // no such overload
let ret = bjs_roundTrip(pointIsSome, pointObjectId)
return Optional<Point>.bridgeJSLiftReturn(ret)

after:

let pointIsSome = point.bridgeJSLowerParameter()
bjs_roundTrip(pointIsSome)
return Optional<Point>.bridgeJSLiftReturn()

The export side already used the stack ABI for optional structs and is unchanged. The change is guarded to the import context, so non-optional struct imports keep their existing object-id ABI.

Adds an end-to-end runtime round-trip test (some and none) and a codegen snapshot covering the new import shape.

[MaxDesiatov]

Thanks!