[llvm-readobj] Fix forward build breakages caused by...

[llvm-readobj] Fix forward build breakages caused by https://reviews.llvm.org/rG33b3c86afab06ad61d46456c85c0b565cfff8287

Change: use std::function instead of function_ref because it's not safe to store a function_ref

(original phab: https://reviews.llvm.org/D116787)