[6.4][SwiftBuild] Regression: WebAssembly runtime crash

### Is it reproducible with SwiftPM command-line tools: `swift build`, `swift test`, `swift package` etc? - [x] Confirmed reproduction steps with SwiftPM CLI. The description text _must_ include reproduction steps with either of command-line SwiftPM commands, `swift build`, `swift test`, `swift package` etc. ### Description Building an ElementaryUI web app (SwiftSDK for WebAssembly) with swift-build on 6.4 causes a runtime crash. "native" build system: works Embedded Swift (wasm SDK): works Full Swift and swift-build -> runtime crasher ### Expected behavior no runtime crash ### Actual behavior `Uncaught RuntimeError: memory access out of bounds` (I also saw `null function` in another project) ### Steps to reproduce clone https://github.com/sliemeobn/swift-build-error-reproducer ```sh swift build \ --swift-sdk swift-6.4.x-DEVELOPMENT-SNAPSHOT-2026-07-16-a_wasm \ --toolset wasm-reactor-toolset.json #--build-system native cp .build/debug/WebApp.wasm ./WebApp.wasm # or ./build.sh npx serve ``` open local page browser ### Swift Package Manager version/commit hash _No response_ ### Swift & OS version (output of `swift --version ; uname -a`) Apple Swift version 6.4-dev (LLVM ae2927a7d243ee6, Swift 371cd3f34758dea) Target: arm64-apple-macosx26.0 Build config: +assertions