Reland "[MC][NFC] Make ELFUniquingMap a StringMap (#95006)" (#95030)

This avoids std::map, which is slow, and uses a StringMap. Section name,
group name, linked-to name and unique id are encoded into the key for
fast lookup.

This gives a measurable performance boost for applications that compile
many small object files (e.g., functions in JIT compilers).

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Now also the second case works properly. That's what happens when you do
that last refactoring without re-running all tests... sorry.