[NFC][libc++] Fixes a compiler warning.

Author: mordante

libcxx/test/tools/clang_tidy_checks/header_exportable_declarations.cpp

@@ -275,7 +275,7 @@ static bool is_module(header_exportable_declarations::FileType file_type) {
 
   case header_exportable_declarations::FileType::Unknown:
     llvm::errs() << "This should be unreachable.\n";
-    break;
+    return false;
   }
 }