[Dependency scanning] Make test order-invariant.

Fixes rdar://problem/62473147, I hope.

Author: DougGregor

test/ScanDependencies/module_deps.swift

@@ -109,8 +109,8 @@ import E
 // CHECK-LABEL: "modulePath": "B.pcm"
 
 // CHECK-NEXT: sourceFiles
-// CHECK-NEXT: B.h
-// CHECK-NEXT: module.modulemap
+// CHECK-DAG: module.modulemap
+// CHECK-DAG: B.h
 
 // CHECK: directDependencies
 // CHECK-NEXT: {