[llvm-readobj] - Remove `reportError(std::error_code EC, StringRef Input)` helper.

We do not need it, std::error_code is used mostly for COFF and
this patch rewrites the calls to use a different overload.

Having reportError(std::error_code EC, ... is excessive by itself,
because API that use error codes actually needs refactoring to
use Error/Expected<> instead.

DIfferential revision: https://reviews.llvm.org/D66521

llvm-svn: 369630