[Explicit Module Builds] Ensure that plugin paths are specified to the dependency scanning invocation

b0a01d3 accidentally broke this by restricting these flags to never get passed when EBM is enabled, as opposed to ensuring they get passed on non-EBM and dependency scanning tasks.

Resolves rdar://154326377

Author: artemcm

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -81,7 +81,7 @@ extension Driver {
       break
     }
 
-    let isPlanJobForExplicitModule = parsedOptions.contains(.driverExplicitModuleBuild)
+    let isPlanJobForExplicitModule = parsedOptions.contains(.driverExplicitModuleBuild) && explicitModulePlanner != nil
     let jobNeedPathRemap: Bool
     // If in ExplicitModuleBuild mode and the dependency graph has been computed, add module
     // dependencies.

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -1986,6 +1986,52 @@ final class ExplicitModuleBuildTests: XCTestCase {
     }
   }
 
+  /// Test the libSwiftScan dependency scanning.
+  func testDependencyScanningPluginFlagPropagation() throws {
+    let (stdlibPath, shimsPath, toolchain, hostTriple) = try getDriverArtifactsForScanning()
+
+    // The dependency oracle wraps an instance of libSwiftScan and ensures thread safety across
+    // queries.
+    let dependencyOracle = InterModuleDependencyOracle()
+    let scanLibPath = try XCTUnwrap(toolchain.lookupSwiftScanLib())
+    try dependencyOracle.verifyOrCreateScannerInstance(swiftScanLibPath: scanLibPath)
+
+    // Create a simple test case.
+    try withTemporaryDirectory { path in
+      let main = path.appending(component: "testDependencyScanning.swift")
+      try localFileSystem.writeFileContents(main, bytes:
+        """
+        import C;\
+        import E;\
+        import G;
+        """
+      )
+
+      let cHeadersPath: AbsolutePath =
+      try testInputsPath.appending(component: "ExplicitModuleBuilds")
+        .appending(component: "CHeaders")
+      let swiftModuleInterfacesPath: AbsolutePath =
+      try testInputsPath.appending(component: "ExplicitModuleBuilds")
+        .appending(component: "Swift")
+      let sdkArgumentsForTesting = (try? Driver.sdkArgumentsForTesting()) ?? []
+      var driver = try Driver(args: ["swiftc",
+                                     "-I", cHeadersPath.nativePathString(escaped: true),
+                                     "-I", swiftModuleInterfacesPath.nativePathString(escaped: true),
+                                     "-I", stdlibPath.nativePathString(escaped: true),
+                                     "-I", shimsPath.nativePathString(escaped: true),
+                                     "/tmp/Foo.o",
+                                     "-plugin-path", "PluginA", "-external-plugin-path", "Plugin~B#Bexe",
+                                     "-explicit-module-build",
+                                     "-working-directory", path.nativePathString(escaped: true),
+                                     "-disable-clang-target",
+                                     main.nativePathString(escaped: true)] + sdkArgumentsForTesting)
+      let resolver = try ArgsResolver(fileSystem: localFileSystem)
+      var scannerCommand = try driver.dependencyScannerInvocationCommand().1.map { try resolver.resolve($0) }
+      XCTAssertTrue(scannerCommand.contains("-plugin-path"))
+      XCTAssertTrue(scannerCommand.contains("-external-plugin-path"))
+    }
+  }
+
   /// Test the libSwiftScan dependency scanning.
   func testDependencyScanning() throws {
     let (stdlibPath, shimsPath, toolchain, hostTriple) = try getDriverArtifactsForScanning()