[llvm-readelf] - Switch to using from `reportWarning` to `reportUniqueWarning` in `DynRegionInfo`.

This is a part of the plan we had previously to convert all calls to
`reportUniqueWarning` and then rename it to just `reportWarning`.

I was a bit unsure about this particular change at first, because it doesn't add a
new functionality: seems it is impossible to trigger a warning duplication currently.

At the same time I find the idea of the plan mentioned very reasonable.
And with that we will be sure that `DynRegionInfo` can't report duplicate
warnings, what looks like a nice feature for possible refactorings and further tool development.

Differential revision: https://reviews.llvm.org/D92224