[CodeGen] Use LLVM_ATTRIBUTE_UNUSED instead of LLVM_DUMP_METHOD on a raw_ostream operator<<.

LLVM_DUMP_METHOD includes ATTRIBUTE_NOINLINE. operator<< isn't
what we normally consider a dump method so it should be ok to inline.

This fixes a warning from gcc that some other declaration for some
other class was inline but this one is noinline. Seems like a bogus
warning from gcc really.