/usr/include/c++/v1/support/ is in libc++-dev 6.0-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/c++/v1/support/android/
/usr/include/c++/v1/support/android/locale_bionic.h
/usr/include/c++/v1/support/fuchsia/
/usr/include/c++/v1/support/fuchsia/xlocale.h
/usr/include/c++/v1/support/ibm/
/usr/include/c++/v1/support/ibm/limits.h
/usr/include/c++/v1/support/ibm/locale_mgmt_aix.h
/usr/include/c++/v1/support/ibm/support.h
/usr/include/c++/v1/support/ibm/xlocale.h
/usr/include/c++/v1/support/musl/
/usr/include/c++/v1/support/musl/xlocale.h
/usr/include/c++/v1/support/newlib/
/usr/include/c++/v1/support/newlib/xlocale.h
/usr/include/c++/v1/support/solaris/
/usr/include/c++/v1/support/solaris/floatingpoint.h
/usr/include/c++/v1/support/solaris/wchar.h
/usr/include/c++/v1/support/solaris/xlocale.h
/usr/include/c++/v1/support/win32/
/usr/include/c++/v1/support/win32/limits_msvc_win32.h
/usr/include/c++/v1/support/win32/locale_win32.h
/usr/include/c++/v1/support/xlocale/
/usr/include/c++/v1/support/xlocale/__nop_locale_mgmt.h
/usr/include/c++/v1/support/xlocale/__posix_l_fallback.h
/usr/include/c++/v1/support/xlocale/__strtonum_fallback.h
/usr/include/c++/v1/support/xlocale/xlocale.h