[llvm-readobj] - Cleanup the DwarfCFIEH::PrinterContext class. NFCI.

It would be nice to switch to `reportUniqueWarnings` from
`reportError` in this class, but first of all it needs a cleanup.

This patch:
1) Eliminates autos.
2) Removes code duplication.
3) Changes how the code works with `Expected<>`.
4) Introduces 2 new `using`s to make the code a bit shorter.

Differential revision: https://reviews.llvm.org/D80726