ARCMigrate: Use hash_combine in the DenseMapInfo for EditEntry

Simplify the DenseMapInfo for `EditEntry` by migrating from
`FoldingSetNodeID` to `llvm::hash_combine`. Besides the cleanup, this
reduces the diff for a future patch which changes the type of one of the
fields.

There should be no real functionality change here, although I imagine
the hash value will churn since its a different hashing infrastructure.

Differential Revision: https://reviews.llvm.org/D92630