Add back propagating '-external-plugin-path' driver agruments

(cherry picked from commit 6ac0b90)

Author: rintaro

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -269,6 +269,10 @@ extension Driver {
       commandLine.appendPath(localPluginPath)
     }
 
+    if isFrontendArgSupported(.externalPluginPath) {
+      try commandLine.appendAll(.externalPluginPath, from: &parsedOptions)
+    }
+
     // Pass down -user-module-version if we are working with a compiler that
     // supports it.
     if let ver = parsedOptions.getLastArgument(.userModuleVersion)?.asSingle,