[cxx-interop] Keep pulling in the entire libc++

libc++ recently split the `std` module into many top-level modules: llvm/llvm-project@571178a

Previously if a C++ module had `#include <iosfwd>`, importing that module in Swift would make the entire C++ stdlib visible from Swift, since it was a single top-level Clang module. After libc++ got split it doesn't automatically do so, but we need to preserve the current behavior for Swift users.

rdar://119270491

Author: egorzhdan

include/swift/ClangImporter/ClangImporter.h

@@ -637,6 +637,11 @@ namespace importer {
 /// Returns true if the given module has a 'cplusplus' requirement.
 bool requiresCPlusPlus(const clang::Module *module);
 
+/// Returns true if the given module is one of the C++ standard library modules.
+/// This could be the top-level std module, or any of the libc++ split modules
+/// (std_vector, std_iosfwd, etc).
+bool isCxxStdModule(const clang::Module *module);
+
 /// Returns the pointee type if the given type is a C++ `const`
 /// reference type, `None` otherwise.
 llvm::Optional<clang::QualType>

lib/ClangImporter/ClangImporter.cpp

@@ -2163,7 +2163,8 @@ ModuleDecl *ClangImporter::Implementation::loadModule(
   ASTContext &ctx = getNameImporter().getContext();
 
   // `CxxStdlib` is the only accepted spelling of the C++ stdlib module name.
-  if (path.front().Item.is("std"))
+  if (path.front().Item.is("std") ||
+      path.front().Item.str().starts_with("std_"))
     return nullptr;
   if (path.front().Item == ctx.Id_CxxStdlib) {
     ImportPath::Builder adjustedPath(ctx.getIdentifier("std"), importLoc);
@@ -3985,6 +3986,13 @@ ModuleDecl *ClangModuleUnit::getOverlayModule() const {
       ImportPath::Module::Builder builder(M->getName());
       (void) owner.loadModule(SourceLoc(), std::move(builder).get());
     }
+    // If this Clang module is a part of the C++ stdlib, and we haven't loaded
+    // the overlay for it so far, it is a split libc++ module (e.g. std_vector).
+    // Load the CxxStdlib overlay explicitly.
+    if (!overlay && importer::isCxxStdModule(clangModule)) {
+      ImportPath::Module::Builder builder(Ctx.Id_CxxStdlib);
+      overlay = owner.loadModule(SourceLoc(), std::move(builder).get());
+    }
     auto mutableThis = const_cast<ClangModuleUnit *>(this);
     mutableThis->overlayModule.setPointerAndInt(overlay, true);
   }
@@ -7624,12 +7632,7 @@ const clang::TypedefType *ClangImporter::getTypeDefForCXXCFOptionsDefinition(
 
 bool importer::requiresCPlusPlus(const clang::Module *module) {
   // The libc++ modulemap doesn't currently declare the requirement.
-  if (module->getTopLevelModuleName() == "std")
-    return true;
-  // In recent libc++ versions the module is split into multiple top-level
-  // modules (std_vector, std_utility, etc).
-  if (module->getTopLevelModule()->IsSystem &&
-      module->getTopLevelModuleName().starts_with("std_"))
+  if (isCxxStdModule(module))
     return true;
 
   // Modulemaps often declare the requirement for the top-level module only.
@@ -7643,6 +7646,18 @@ bool importer::requiresCPlusPlus(const clang::Module *module) {
   });
 }
 
+bool importer::isCxxStdModule(const clang::Module *module) {
+  if (module->getTopLevelModuleName() == "std")
+    return true;
+  // In recent libc++ versions the module is split into multiple top-level
+  // modules (std_vector, std_utility, etc).
+  if (module->getTopLevelModule()->IsSystem &&
+      module->getTopLevelModuleName().starts_with("std_"))
+    return true;
+
+  return false;
+}
+
 llvm::Optional<clang::QualType>
 importer::getCxxReferencePointeeTypeOrNone(const clang::Type *type) {
   if (type->isReferenceType())

lib/ClangImporter/ImportDecl.cpp

@@ -3389,10 +3389,8 @@ namespace {
                     ->getTopLevelModule()
                     ->getFullModuleName() == n;
         };
-        if (topLevelModuleEq(decl, "std") ||
-            (decl->getOwningModule() && decl->getOwningModule()->IsSystem &&
-             StringRef(decl->getOwningModule()->getTopLevelModule()->Name)
-                 .starts_with("std_"))) {
+        if ((decl->getOwningModule() &&
+             importer::isCxxStdModule(decl->getOwningModule()))) {
           if (isAlternativeCStdlibFunctionFromTextualHeader(decl)) {
             return nullptr;
           }