40906897: New Build System: Cannot clean app if custom build directory is used

[openradar-mirror]

Description

Area:
Xcode

Summary:
When attempting to clean an app build with the new build system enabled, the app build aborts with an error.

Steps to Reproduce:

1. Create a new app
2. Specify a custom build directory via Build Settings
3. Try to clean the project

Expected Results:
The build succeeds.

Actual Results:
The build fails with the following error:

[ERROR] note: Using new build system
[ERROR] error: Could not delete /path/to/project/build because it was not created by the build system and it is not a subfolder of derived data.
[ERROR] ** CLEAN FAILED **

Version/Build:
Version 10.0 beta (10L176w)

Configuration:
iOS 12.0 Simulator, new build system enabled

Product Version: iOS 12.0 + Xcode 10
Created: 2018-06-07T23:30:57.371670
Originated: 2018-06-07T00:00:00
Open Radar Link: http://www.openradar.me/40906897