[llvm-readelf/obj] - Cleanup the code. NFCI.

This:
1) Replaces pointers with references in many places.
2) Adds few TODOs about fixing possible unhandled errors (in ARMEHABIPrinter.h).
3) Replaces `auto`s with actual types.
4) Removes excessive arguments.
5) Adds `const ELFFile<ELFT> &Obj;` member to `ELFDumper` to simplify the code.

Differential revision: https://reviews.llvm.org/D88097