[Dependency Scanning] Treat '-embed-tbd-for-module' modules as implicit import dependencies.

Otherwise the scanning action will not look for them as dependencies, and the compilation it is used to inform will not specify these moduels as explicit inpouts.

Resolves rdar://104761392

Author: artemcm

lib/DependencyScan/ScanDependencies.cpp

@@ -1195,6 +1195,13 @@ identifyMainModuleDependencies(CompilerInstance &instance) {
       mainDependencies.addModuleImport(mainModule->getName().str(),
                                            &alreadyAddedModules);
     }
+
+    // All modules specified with `-embed-tbd-for-module` are treated as implicit
+    // dependnecies for this compilation since they are not guaranteed to be impored
+    // in the source.
+    for (const auto &tbdSymbolModule : instance.getInvocation().getTBDGenOptions().embedSymbolsFromModules) {
+      mainDependencies.addModuleImport(tbdSymbolModule, &alreadyAddedModules);
+    }
   }
 
   return mainDependencies;

test/ScanDependencies/embed_tbd_module_dependency.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: mkdir -p %t/clang-module-cache
+
+// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/clang-module-cache %s -o %t/deps.json -I %S/Inputs/CHeaders -I %S/Inputs/Swift -swift-version 4 -embed-tbd-for-module E
+// RUN: %FileCheck %s < %t/deps.json
+
+// CHECK: "mainModuleName": "deps"
+// CHECK-NEXT:  "modules": [
+// CHECK-NEXT:    {
+// CHECK-NEXT:      "swift": "deps"
+// CHECK-NEXT:    },
+// CHECK:      "directDependencies": [
+
+// Ensure there is a dependency on 'E' even though it is not explicitly imported
+// CHECK:          "swift": "E"
+