[llvm-readobj] - Simplify the code.

We have a Field struct which has a StringRef member Str.

The code needs to create and keep alive the temporarily
std::string variables because of that.
That is not convenient and makes the code be more complicated
than it could be.

There seems to be no reason to keep Str be StringRef.
The patch changes it to be std::string and
rearranges the code around slightly.

Differential revision: https://reviews.llvm.org/D57447

llvm-svn: 352623