[lib/Object] - Make ELFObjectFile::getSymbol() return Expected<>.

This was requested in comments for D93209:
https://reviews.llvm.org/D93209#inline-871192

D93209 fixes an issue with `ELFFile<ELFT>::getEntry`,
after what `getSymbol` starts calling `report_fatal_error` for previously
missed invalid cases.

This patch makes it return `Expected<>` and updates callers.
For few of them I had to add new `report_fatal_error` calls. But I see no
way to avoid it currently. The change would affects too many places, e.g:
`getSymbolBinding` and other methods are used from `ELFSymbolRef`
which is used in too many places across LLVM.

Differential revision: https://reviews.llvm.org/D93297