[Sema] Do not emit -Wunused-variable for variables declared with cleanup attribute

A variable declared with __attribute__((cleanup)) cannot be unused, as
its address is passed to the clean up function. Do not emit
-Wunused-variable for variables declared with the cleanup attribute,
which matches GCC's behavior: https://godbolt.org/z/dz5YfTsan

Reviewed By: erichkeane, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D152180