WebAssembly: Copy clang headers from target toolchain (#86)

The clang headers were copied from host toolchain at the first "rsync"
step. But host toolchain can be optional, so we should copy the headers
from the target toolchain instead.

Author: kateinoigakukun

Sources/SwiftSDKGenerator/SwiftSDKRecipes/WebAssemblyRecipe.swift

@@ -105,7 +105,9 @@ public struct WebAssemblyRecipe: SwiftSDKRecipe {
     logGenerationStep("Copying Swift core libraries for the target triple into Swift SDK bundle...")
     for (pathWithinPackage, pathWithinSwiftSDK) in [
       ("clang", pathsConfiguration.toolchainDirPath.appending("usr/lib")),
+      ("swift/clang", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift")),
       ("swift/wasi", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift")),
+      ("swift_static/clang", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift_static")),
       ("swift_static/wasi", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift_static")),
       ("swift_static/shims", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift_static")),
       ("swift_static/CoreFoundation", pathsConfiguration.toolchainDirPath.appending("usr/lib/swift_static")),