[llvm-readobj] - Use cantFail() for all `Obj->sections()` calls. NFCI.

`ELFDumper<ELFT>::ELFDumper` calls `Obj->sections()` in its constructor:
https://github.com/llvm/llvm-project/blob/master/llvm/tools/llvm-readobj/ELFDumper.cpp#L2046

this means that all subsequent calls can't fail and can be
wrapped into `cantFail` in instead of `unwrapOrError` for simplicity.

Actually we already do it in a few places. In this patch I've fixed all
other places I've found.

Differential revision: https://reviews.llvm.org/D83126