[llvm-readobj] - Refactor how the code dumps relocations.

There is a strange "feature" of the code: it handles all relocations as `Elf_Rela`.
For handling `Elf_Rel` it converts them to `Elf_Rela` and passes `bool IsRela` to
specify the real type everywhere.

A related issue is that the
`decode_relrs` helper in lib/Object has to return `Expected<std::vector<Elf_Rela>>`
because of that, though it could return a vector of `Elf_Rel`.

I think we should just start using templates for relocation types, it makes the code
cleaner and shorter. This patch does it.

Differential revision: https://reviews.llvm.org/D83871