swiftwasm#752)

Make Data typed-array constructor non-optional

Optional @js structs could not be used as parameters or return values of
imported (@JSFunction) signatures: the generator lowered Optional<Struct>
using the non-optional object-id ABI ([isSome, objectId] / a single Int32
return), for which no Optional lowering exists, so the generated thunk did
not compile.

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

Adds a jsRoundTripOptionalPoint runtime round-trip (some + none) and a
SwiftStructImports codegen snapshot.

…-import

BridgeJS: Support optional @js struct in imported function signatures

Case enums (enums without raw values or associated values) already bridged
across the export boundary as their Int32 tag, but the TypeScript import path
rejected them with "Enum types are not yet supported in TypeScript imports".
The JS glue already round-trips the tag in both directions, so enable case
enums as imported (@JSFunction) parameters and return values by lowering and
lifting that Int32 tag in the import context, matching the export side.

Adds a CaseEnumImports round-trip test and an EnumCaseImport codegen snapshot.

BridgeJS: Support case enums as imported function parameters and returns

A Wasm i64 parameter or return value is represented as a JavaScript BigInt.
The generated JS used a plain 0 as the placeholder for the absent case of an
optional i64 parameter (isSome ? v : 0) and for the error-path return of an
imported thunk, so calling such an export with null (or an imported i64
function throwing) raised "TypeError: Cannot convert 0 to a BigInt".

Emit 0n for i64 in both placeholders (jsZeroLiteral and the imported-thunk
return placeholder). This was latent because the optional Int64/UInt64
round-trip tests never exercised the none case; add those assertions.

…-zero-literal