Exempt C++ namespaces from internal-import checking

C++ namespaces always get imported into the bridging header's module
for Reasons. Exempt them from internal-import checking, since we get
checking for the namespace members that are actually used.

lib/Sema/TypeCheckAccess.cpp

@@ -2694,6 +2694,11 @@ void swift::recordRequiredImportAccessLevelForDecl(
   if (definingModule == dc->getParentModule())
     return;
 
+  // Egregious hack: if the declaration is for a C++ namespace, assume it's
+  // accessible.
+  if (isa_and_nonnull<clang::NamespaceDecl>(decl->getClangDecl()))
+    return;
+
   sf->registerRequiredAccessLevelForModule(definingModule, accessLevel);
 
   if (auto attributedImport = sf->getImportAccessLevel(definingModule)) {

test/ClangImporter/Inputs/cxx-bridging-header.h

@@ -0,0 +1,9 @@
+
+namespace OuterNS {
+  class MyPoint {
+  public:
+    double x, y;
+  };
+}
+
+#include "cxx-outer-ns.h"

test/ClangImporter/Inputs/cxx-outer-ns.h

@@ -0,0 +1,7 @@
+namespace OuterNS {
+  enum Color {
+    red,
+    green,
+    blue
+  };
+}

test/ClangImporter/Inputs/module.modulemap

@@ -0,0 +1,3 @@
+module CXXOuterNS {
+  header "cxx-outer-ns.h"
+}

test/ClangImporter/InternalBridgingHeader/access_checking_cxx.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -internal-import-bridging-header %S/../Inputs/cxx-bridging-header.h -sdk %clang-importer-sdk -cxx-interoperability-mode=default -I %S/../Inputs
+
+public func getRed() -> OuterNS.Color { OuterNS.red }
+// expected-error@-1{{function cannot be declared public because its result uses an internal type}}
+// expected-note@-2{{enum 'OuterNS' is imported by this file as 'internal' from bridging header}}
+
+public func getX(point: OuterNS.MyPoint) -> Double { point.x }
+// expected-error@-1{{function cannot be declared public because its parameter uses an internal type}}
+// expected-note@-2{{enum 'OuterNS' is imported by this file as 'internal' from bridging header}}