Fix subcommand discovery

Author: stevapple

Sources/swift-driver/main.swift

@@ -30,11 +30,16 @@ do {
   if case .subcommand(let subcommand) = mode {
     // We are running as a subcommand, try to find the subcommand adjacent to the executable we are running as.
     // If we didn't find the tool there, let the OS search for it.
-    let subcommandPath = Process.findExecutable(arguments[0])?.parentDirectory.appending(component: subcommand)
-                         ?? Process.findExecutable(subcommand)
+    #if os(Windows)
+    let filename = subcommand + ".exe"
+    #else
+    let filename = subcommand
+    #endif
+    let subcommandPath = Process.findExecutable(arguments[0])?.parentDirectory.appending(component: filename)
+                         ?? Process.findExecutable(filename)
 
     if subcommandPath == nil || !localFileSystem.exists(subcommandPath!) {
-      fatalError("cannot find subcommand executable '\(subcommand)'")
+      fatalError("cannot find subcommand executable '\(filename)'")
     }
 
     // Execute the subcommand.