[llvm] [llvm][metadata][NFC] Minimize local variable scopes (PR #68438)

[Paul Kirth via llvm-commits]

ilovepi wrote:

huh, not sure why this was working for me locally. I'm guessing maybe there is an issue w/ my build flags. 

Likely, I'll roll back the change to std::pair and re-introduce the DenseMapInfo specialization.

https://github.com/llvm/llvm-project/pull/68438