2018.1.0b13 - was fine at first, then added bugsnag sdk and this happened.
#include <stdbool.h> solved it for me, thanks guys ^_^

Please reopen! This is still occuring in Unity 2018.1.6f1.

Having the issue in Unity 2018.1.4f1 as well. Adding #include <stdbool.h> solved it as well.

However, the bool is only needed for some specific methods, a WWW-related method in my case. Probably only the "more-likely-depricated methods" are bools in these files?

So probably everyone having the problem and planning to update the project in future: Check which methods exactly use them and think about using other ones.

Nevertheless this IS a bug and should be fixed (again).

Automatic solution for cloud build and because it's more comfortable:

public class PostProcessBuild
{
[PostProcessBuild]
public static void OnPostProcessBuild(BuildTarget target, string path)
{
#if UNITY_IPHONE
string unityInterfacePath = path + "/Classes/Unity/UnityInterface.h";
if (File.Exists(unityInterfacePath))
{
List<string> interfaceLines = File.ReadAllLines(unityInterfacePath).ToList();
interfaceLines.Insert(0, "#include <stdbool.h>");
StringBuilder sb = new StringBuilder();
foreach (string s in interfaceLines)
{
sb.Append(s);
sb.Append("\n");
}
File.WriteAllText(unityInterfacePath, sb.ToString());
}
else
{
Debug.LogError("UnityInterface.h doesn't exist");
}
#endif
}
}

Having issue in 2018.1.1f1 also ...

Having this issue in 2018.1.1f1.

regression in 2018.1.0f2

Still happening in 2017.2.0f1. Fixable by adding #include <stdbool.h> to the top of UnityRendering.h and UnityInterface.h