[ADT] Allow llvm::hash_code as DenseMap key.

[sammccall]

Summary

This is useful when hash collisions are unlikely and acceptable, e.g. in clangd
completion ranking.

[ioeric]

lgtm

include/llvm/ADT/DenseMapInfo.h

static bool isEqual(ArrayRef<T> LHS, ArrayRef<T> RHS) {
     if (RHS.data() == getEmptyKey().data())
       return LHS.data() == getEmptyKey().data();
     if (RHS.data() == getTombstoneKey().data())
       return LHS.data() == getTombstoneKey().data();
     return LHS == RHS;
   }
 };
 
+template <> struct DenseMapInfo<hash_code> {
+  static inline hash_code getEmptyKey() { return hash_code(-1); }
+  static inline hash_code getTombstoneKey() { return hash_code(-2); }
+  static unsigned getHashValue(hash_code val) { return val; }
+  static bool isEqual(hash_code LHS, hash_code RHS) { return LHS == RHS; }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_ADT_DENSEMAPINFO_H