[Object/COFF.h] - Stop returning std::error_code in a few methods. NFCI.

[Object/COFF.h] - Stop returning std::error_code in a few methods. NFCI.

There are 4 methods that return std::error_code now,
though they do not have to because they are always succeed.
I refactored them.

This allows to simplify the code in tools a bit.

llvm-svn: 369263