[llvm-readobj] - Remove deprecated unwrapOrError(Expected<T> EO).

[llvm-readobj] - Remove deprecated unwrapOrError(Expected<T> EO).

This patch changes the code to use a modern unwrapOrError(StringRef Input, Expected<T> EO)
version that contains the input source name and removes the deprecated version.

Differential revision: https://reviews.llvm.org/D65946

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368428 91177308-0d34-0410-b5e6-96231b3b80d8