[llvm] [llvm] Replace unordered_{map, set} with Dense{Map, Set} (PR #202222)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Hmm. We can do this, although DenseSet<ptr> is probably better served by SmallPtrSet. But the value proposition is rather low (unless we completely ban std::unordered_* and enforce this).

https://github.com/llvm/llvm-project/pull/202222