[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`.

Author: bnbarham

include/swift/Basic/APIntMap.h

@@ -38,11 +38,11 @@ struct WidthPreservingAPIntDenseMapInfo {
   }
 
   static unsigned getHashValue(const APInt &Key) {
-    return static_cast<unsigned>(hash_value(Key));
+    return llvm::DenseMapInfo<APInt>::getHashValue(Key);
   }
 
   static bool isEqual(const APInt &LHS, const APInt &RHS) {
-    return LHS.getBitWidth() == RHS.getBitWidth() && LHS == RHS;
+    return llvm::DenseMapInfo<APInt>::isEqual(LHS, RHS);
   }
 };