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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> 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).

Thanks for the review!  I've left "heterogenous lookup" in the title because it is a concise term used often in the C++ community, but I realize some people may not be familiar with the concept.  I expanded a bit in the PR description.

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