How to reproduce:
1. Open the attached "Post-Processing Package compile failure.zip" project
2. Open Package Manager window
3. Make sure that the project has "Post-Processing" package is installed
4. Disable the built-in "Unity Analytics" package

Alternative scenario:
1. Create a new project
2. Create an empty script
3. Open the script and add the following code line: "bool b = XRSettings.enabled;"
4. Add a "using" directive: "using UnityEngine.XR;"
5. Save the script
6. Back in Unity Editor, open Package Manager window
7. Disable the built-in "Unity Analytics" package

Expected result: Disabling built-in "Unity Analytics" package does not cause compilation errors when "UnityEngine.XR" namespace is used in the project.
Actual result: If a project is using "UnityEngine.XR" namespace, compilation errors are caused after disabling "Unity Analytics" package.

Reproducible with - 2018.2.11f1, 2018.2.13f1, 2018.3.0b3, 2018.3.0b7, 2019.1.0a1, 2019.1.0a5
Not reproducible with - 2018.2.10f1, 2018.3.0b2

Error thrown (main scenario):
error CS1070: The type `UnityEngine.XR.XRSettings' has been forwarded to an assembly that is not referenced. Enable the built in package 'Unity Analytics' in the Package Manager window to fix this error.

Fixed in 2019.1.0a13
Backported to 2018.3.4f1