clang-tidy: Enable -misc-unused-using-decls check
It is supported at least from clang-format-8: https://prereleases.llvm.org/8.0.0/rc4/tools/clang/tools/extra/docs/clang-tidy/checks/misc-unused-using-decls.html.
It is supported at least from clang-format-8: https://prereleases.llvm.org/8.0.0/rc4/tools/clang/tools/extra/docs/clang-tidy/checks/misc-unused-using-decls.html.