[TextAPI][elfabi] Make TBE handlers functions that return Errors

Since TBEHandler doesn't maintain state or otherwise have any need to be
a class right now, the read and write functions have been moved out and
turned into standalone functions. Additionally, the TBE read function
has been updated to return an Expected value for better error handling.
Tests have been updated to reflect these changes.

Differential Revision: https://reviews.llvm.org/D55450

llvm-svn: 348735