[cxx-interop] Check for type completeness consistently

`clangSema.isCompleteType` checks for decl visibility according to the module visibility rules, which we don't actually need. What we need to check is whether the record has a definition – this is the check we already use elsewhere in ClangImporter.

This makes sure that we can import `std::function` on Windows.

rdar://103979602

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -2959,9 +2959,7 @@ namespace {
         if (notInstantiated)
           return nullptr;
       }
-      if (!clangSema.isCompleteType(
-              decl->getLocation(),
-              Impl.getClangASTContext().getRecordType(decl))) {
+      if (!decl->getDefinition()) {
         // If we got nullptr definition now it means the type is not complete.
         // We don't import incomplete types.
         return nullptr;