Temporarily disable unused-but-set-variable until it's fixed in Clang
Temporarily disable unused-but-set-variable until it's fixed in Clang

`_dispatch_preemption_yield(++spins)` expands to `(void)++spins;` on
various platforms. A recent Clang change updated the
`unused-but-set-variable` warning to skip counting operators as a use,
but it ignores `(void)`. Disable the error until that's fixed.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch