[llvm-readelf/obj] - Move unique warning handling logic to the `ObjDumper`.

[llvm-readelf/obj] - Move unique warning handling logic to the `ObjDumper`.

This moves the `reportUniqueWarning` method to the base class.

My motivation is the following:
I've experimented with replacing `reportWarning` calls with `reportUniqueWarning`
in ELF dumper. I've found that for example for removing them from `DynRegionInfo` helper
class, it is worth to pass a dumper instance to it (to be able to call dumper()->reportUniqueWarning()).
The problem was that `ELFDumper<ELFT>` is a template class. I had to make `DynRegionInfo` to be templated
and do lots of minor changes everywhere what did not look reasonable/nice.

At the same time I guess one day other dumpers like COFF/MachO/Wasm etc might want to
start using `reportUniqueWarning` API too. Then it looks reasonable to move the logic to the
base class.

With that the problem of passing the dumper instance will be gone.

Differential revision: https://reviews.llvm.org/D92218