[NFC][llvm-readobj] Refactor unique warning handler

The unique warning handler was previously a property of the dump style,
but it is commonly used in the dumper too. Since the two ELF output
styles have no impact on the way warnings are printed, this patch moves
the handler and related functions into the dumper class, instead of the
dump style class.

Reviewed by: MaskRay, grimar

Differential Revision: https://reviews.llvm.org/D76777