clang-tidy: disable modernize-concat-nested-namespaces

Concatenated namespaces require C++17, but we still support C++11.

.clang-tidy

@@ -29,6 +29,7 @@ misc-*,\
 -misc-use-internal-linkage,\
 modernize-*,\
 -modernize-avoid-c-arrays,\
+-modernize-concat-nested-namespaces,\
 -modernize-macro-to-enum,\
 -modernize-return-braced-init-list,\
 -modernize-type-traits,\