Prefer llvm-ar over ar on WebAssembly

Static "ar"chivers not prefixed with "llvm-" rarely support WebAssembly
object file formats, and the legacy driver uses "llvm-ar", so it would
be better to keep the consistency.

Author: kateinoigakukun

Sources/SwiftDriver/Toolchains/WebAssemblyToolchain.swift

@@ -95,10 +95,7 @@ public final class WebAssemblyToolchain: Toolchain {
     switch tool {
     case .swiftCompiler:
       return try lookup(executable: "swift-frontend")
-    case .staticLinker(nil):
-      return try lookup(executable: "ar")
-    case .staticLinker(.llvmFull),
-         .staticLinker(.llvmThin):
+    case .staticLinker:
       return try lookup(executable: "llvm-ar")
     case .dynamicLinker:
       // FIXME: This needs to look in the tools_directory first.