This flag, which was introduced due to #17798, is only understood by
Clang and consequently throws warnings on platforms using gcc. Sadly,
there is no good way to treat such warnings as non-fatal with `-Werror`
so for now we simply make this flag specific to platforms known to use
Clang and case-insensitive filesystems (Darwin and Windows).

See #23577.

(cherry picked from commit 3a09b789)