Disable adding '-external-plugin-path' in integrated driver mode

rdar://109060298
(cherry picked from commit bf2de71)

Author: rintaro

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -376,6 +376,7 @@ public final class DarwinToolchain: Toolchain {
     // Pass -external-plugin-path if the current toolchain is not a Xcode
     // default toolchain.
     if
+      !driver.integratedDriver,
       driver.isFrontendArgSupported(.externalPluginPath),
       let xcodeDir = try self.findCurrentSelectedXcodeDir(),
       try !self.executableDir.isDescendant(of: xcodeDir),