[llvm] [NFC] Add a specialization of DenseMapInfo for SmallVector (PR #140380)

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

Thanks for the quick review!

An lld test failed after this landed, something about reading debug info. Checking the builder shows that test failing multiple times on previous builds and the revert before this also looks unrelated to lld. I think unrelated to this change. Will keep an eye on the other builders.

https://github.com/llvm/llvm-project/pull/140380