[cxx-interop] Implicitly import Cxx module

This lifts the requirement for the user to explicitly add `import Cxx` in Swift code that relies on protocols from the `Cxx` module, such as `CxxSequence`.

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -3269,19 +3269,6 @@ static void getImportDecls(ClangModuleUnit *ClangUnit, const clang::Module *M,
     auto *ID = createImportDecl(Ctx, ClangUnit, ImportedMod, Exported);
     Results.push_back(ID);
   }
-
-  if (Ctx.LangOpts.EnableCXXInterop && requiresCPlusPlus(M)) {
-    // Try to load the Cxx module. We don't use it directly here, but we need to
-    // make sure that ASTContext has loaded this module.
-    auto *cxxModule = Ctx.getModuleByIdentifier(Ctx.Id_Cxx);
-    if (cxxModule) {
-      ImportPath::Builder builder(Ctx.Id_Cxx);
-      auto *importCxx =
-          ImportDecl::create(Ctx, ClangUnit, SourceLoc(), ImportKind::Module,
-                             SourceLoc(), builder.get());
-      Results.push_back(importCxx);
-    }
-  }
 }
 
 void ClangModuleUnit::getDisplayDecls(SmallVectorImpl<Decl*> &results, bool recursive) const {
@@ -3918,6 +3905,17 @@ void ClangModuleUnit::getImportedModulesForLookup(
   } else {
     clangModule->getExportedModules(imported);
     topLevel = clangModule->getTopLevelModule();
+
+    // If this is a C++ module, implicitly import the Cxx module, which contains
+    // definitions of Swift protocols that C++ types might conform to, such as
+    // CxxSequence.
+    if (owner.SwiftContext.LangOpts.EnableCXXInterop &&
+        requiresCPlusPlus(clangModule) && clangModule->Name != "CxxShim") {
+      auto *cxxModule =
+          owner.SwiftContext.getModuleByIdentifier(owner.SwiftContext.Id_Cxx);
+      if (cxxModule)
+        imports.push_back({ImportPath::Access(), cxxModule});
+    }
   }
 
   if (imported.empty()) {

test/Interop/Cxx/stdlib/overlay/custom-sequence-typechecker.swift

@@ -1,7 +1,6 @@
 // RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop
 
 import CustomSequence
-import Cxx
 
 func checkIntSequence<S>(_ seq: S) where S: Sequence, S.Element == Int32 {
   let contains = seq.contains(where: { $0 == 3 })

test/Interop/Cxx/stdlib/overlay/custom-sequence.swift

@@ -5,7 +5,6 @@
 
 import StdlibUnittest
 import CustomSequence
-import Cxx
 
 var CxxSequenceTestSuite = TestSuite("CxxSequence")