[llvm] [llvm-readobj] Use heterogenous lookups with std::map (NFC) (PR #114929)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

Functionally LGTM, but the commit title is unhelpful, in my opinion. I'd suggest adding a description of "Support lookups in a section name std::map with both std::string and StringRef" or something to that effect (I'd suggest this be the title, but it's too long probably, so the description will have to do).

https://github.com/llvm/llvm-project/pull/114929