[cxx-interop] Workaround name lookup issues with namespace os

On Apple platforms, a system module `os` declares a `namespace os` under `#if defined(__cplusplus)`. This causes ClangImporter to import it as `enum os` when C++ interop is enabled. This causes name lookup ambiguity (module os vs namespace os) which is resolved in namespace's favor, breaking existing usages.

rdar://119044493

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -1158,6 +1158,14 @@ namespace {
       // Do not import namespace declarations marked as 'swift_private'.
       if (decl->hasAttr<clang::SwiftPrivateAttr>())
         return nullptr;
+      // Workaround for os module declaring `namespace os` on Darwin, causing
+      // name lookup issues. That namespace only declares utility functions that
+      // are not supposed to be used from Swift, so let's just not import the
+      // namespace (rdar://119044493).
+      if (decl->getIdentifier() && decl->getName() == "os" &&
+          decl->getOwningModule() &&
+          decl->getOwningModule()->getTopLevelModuleName() == "os")
+        return nullptr;
       // If this is a top-level namespace, don't put it in the module we're
       // importing, put it in the "__ObjC" module that is implicitly imported.
       if (!decl->getParent()->isNamespace())

test/Interop/Cxx/objc-correctness/os-logger.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs -cxx-interoperability-mode=default %s
+
+// REQUIRES: objc_interop
+// REQUIRES: VENDOR=apple
+
+import os
+
+var _: os.Logger! = nil