SwiftDriver: explicitly select RSP handling on Windows

When cross-compiling from Windows to Android, if we spill past the
command line limit, we would need to emit a response file. The response
file is emitted into the style of the build rather than the host. This
means that on Unix platforms, we need to indicate that the response file
is Windows not Unix.

We rely on the internal behaviour of the second flag being special cased
and emit the `--rsp-quoting=windows` parameter in the second argument
position to ensure that it is passed on the command line (as if the
argument is pushed into the response file, it will not be possible to
honour it for processing the file).

Author: compnerd

Sources/SwiftDriver/Jobs/GenericUnixToolchain+LinkerSupport.swift

@@ -44,6 +44,10 @@ extension GenericUnixToolchain {
     sanitizers: Set<Sanitizer>,
     targetInfo: FrontendTargetInfo
   ) throws -> ResolvedTool {
+#if os(Windows)
+    commandLine.appendFlag("--rsp-quoting=windows")
+#endif
+
     let targetTriple = targetInfo.target.triple
     switch linkerOutputType {
     case .dynamicLibrary: