build: Build Swift SDK for WebAssembly
This change adds a new product, WasmSwiftSDK, to build the Swift SDK for
WebAssembly. The product is built using the swift-sdk-generator package
and takes just built WebAssembly stdlib, clang runtime libraries,
and wasi-sysroot as input, and produces a Swift SDK artifactbundle under
swift-sdk-generator/Bundles.
