[llvm-readobj/elf] - Simplify and refine the implementation which dumps .stack_sizes

Our implementation of stack sizes section dumping heavily uses `ELFObjectFile<ELFT>`,
while the rest of the code uses `ELFFile<ELFT>`.

That APIs are very different. `ELFObjectFile<ELFT>` is very generic
and has `SectionRef`, `RelocationRef`, `SymbolRef` and other generic concepts.
The `ELFFile<ELFT>` class works directly with `Elf_Shdr`, `Elf_Rel[a]`, `Elf_Sym` etc,
what is probably much cleaner for ELF dumper.

Also, `ELFObjectFile<ELFT>` API does not always provide a way to check
for possible errors. E.g. the implementation of `symbol_end()` does not verify the `sh_size`:

```
template <class ELFT>
basic_symbol_iterator ELFObjectFile<ELFT>::symbol_end() const {
  const Elf_Shdr *SymTab = DotSymtabSec;
  if (!SymTab)
    return symbol_begin();
  DataRefImpl Sym = toDRI(SymTab, SymTab->sh_size / sizeof(Elf_Sym));
  return basic_symbol_iterator(SymbolRef(Sym, this));
}
```
There are many other examples which makes me thing we might win from
switching to `ELFFile<ELFT>` API, where we heavily validate an input data already.

This patch is the first step in this direction. I've converted the large portion of the code
to use `ELFFile<ELFT>`.

Differential revision: https://reviews.llvm.org/D87362

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