[rebranch] Delegate to DenseMapInfo<APInt> where possible
[rebranch] Delegate to DenseMapInfo<APInt> where possible

`WidthPreservingAPIntDenseMapInfo` uses the same `getHashValue` and
`isEqual` as the normal `DenseMapInfo` but with different empty and
tombstone keys. `hash_value` is no longer a complete type, so just
delegate to `DenseMapInfo<APInt>` rather than add another `#include`.

apple/swift,roambotics/swift,apple/swift,benlangmuir/swift,glessard/swift,apple/swift,benlangmuir/swift,apple/swift,glessard/swift,glessard/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,roambotics/swift,apple/swift,glessard/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,roambotics/swift