[Dependency Scanning] On discovering cross-import overlays, do not add their discovered dependencies to the main module's direct dependencies

Instead, only add the overlay itself, and let it refer to its own dependencies, which will still get recorded in the overall output.

Resolves rdar://117010118

Author: artemcm

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -854,12 +854,22 @@ void ModuleDependencyScanner::discoverCrossImportOverlayDependencies(
   }
 
   // Update main module's dependencies to include these new overlays.
+  auto resolvedDummyDep =
+      *(cache.findDependency(dummyMainName, ModuleDependencyKind::SwiftSource)
+            .value());
   auto mainDep =
       *(cache.findDependency(mainModuleName, ModuleDependencyKind::SwiftSource)
             .value());
-  std::for_each(/* +1 to exclude dummy main*/ allModules.begin() + 1,
-                allModules.end(), [&](ModuleDependencyID dependencyID) {
-                  mainDep.addModuleDependency(dependencyID);
+  auto newOverlayDeps = resolvedDummyDep.getDirectModuleDependencies();
+  auto existingMainDeps = mainDep.getDirectModuleDependencies();
+  ModuleDependencyIDSet existingMainDepsSet(existingMainDeps.begin(),
+                                            existingMainDeps.end());
+  // Ensure we do not add cross-import overlay dependencies in case they
+  // were already explicitly imported
+  std::for_each(newOverlayDeps.begin(), newOverlayDeps.end(),
+                [&](ModuleDependencyID crossImportOverlayModID) {
+                  if (!existingMainDepsSet.count(crossImportOverlayModID))
+                    mainDep.addModuleDependency(crossImportOverlayModID);
                 });
   cache.updateDependency(
       {mainModuleName.str(), ModuleDependencyKind::SwiftSource}, mainDep);

test/ScanDependencies/Inputs/Swift/_cross_import_E.swiftinterface

@@ -3,4 +3,5 @@
 import Swift
 import E
 import SubE
+import X
 public func funcCrossImportE() {}

test/ScanDependencies/module_deps_cross_import_overlay.swift

@@ -24,4 +24,9 @@ import SubEWrapper
 // CHECK-DAG:   "swift": "_StringProcessing"
 // CHECK-DAG:   "swift": "_cross_import_E"
 // CHECK-DAG:   "clang": "_SwiftConcurrencyShims"
+// Ensure a transitive dependency via "_cross_import_E" is not a direct dep of main module
+// CHECK-NOT:   "clang": "X"
 // CHECK: ],
+
+// Ensure a transitive dependency via "_cross_import_E" is recorded in the graph still
+// CHECK:   "clang": "X"