[Dependency Scannning] Handle special case import of Clang Private "submodules"

There is a special case that already exists in 'ClangImporter' for implicit module loading:
Import of a "submodule" named "Foo.Private" is treated as a top-level module named "Foo_Private".
Clang has special support for this.

Resolves rdar://108287140

Author: artemcm

include/swift/AST/ModuleDependencies.h

@@ -543,7 +543,18 @@ class ModuleDependencyInfo {
   /// Add a dependency on the given module, if it was not already in the set.
   void addModuleImport(ImportPath::Module module,
                        llvm::StringSet<> *alreadyAddedModules = nullptr) {
-    addModuleImport(module.front().Item.str(), alreadyAddedModules);
+    std::string ImportedModuleName = module.front().Item.str().str();
+    auto submodulePath = module.getSubmodulePath();
+    if (submodulePath.size() > 0 && !submodulePath[0].Item.empty()) {
+      assert(submodulePath.size() == 1 && "Unsupported Clang submodule import");
+      auto submoduleComponent = submodulePath[0];
+      // Special case: a submodule named "Foo.Private" can be moved to a top-level
+      // module named "Foo_Private". ClangImporter has special support for this.
+      if (submoduleComponent.Item.str() == "Private")
+        ImportedModuleName = ImportedModuleName + "_Private";
+    }
+
+    addModuleImport(ImportedModuleName, alreadyAddedModules);
   }
 
   /// Add all of the module imports in the given source

test/ScanDependencies/Inputs/CHeaders/X_Private.h

@@ -0,0 +1 @@
+void funcXPrivate(void);

test/ScanDependencies/Inputs/CHeaders/module.modulemap

@@ -42,3 +42,7 @@ module X {
   header "X.h"
   export *
 }
+module X_Private {
+  header "X_Private.h"
+  export *
+}

test/ScanDependencies/module_deps_clang_private_submodule.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t.module-cache)
+// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t.module-cache %s -o %t.deps.json -I %S/Inputs/CHeaders
+
+// RUN: %FileCheck %s < %t.deps.json
+// CHECK: "clang": "X_Private"
+import X.Private
+