Feature Request: Show warning during iOS build with missing .xcconfig

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Yes

Tell us about the problem

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Which platform(s) does your issue occur on?

iOS

Please provide the following version numbers that your issue occurs with:

• CLI: 4.2.0-2018-07-12-11970 (next)
  Project is using 4.1.0

Please tell us how to recreate the issue in as much detail as possible.

At some time in previous release, maybe NS 2 or 3x projects provided a build.xcconfig file which specifies:

ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
ASSETCATALOG_COMPILER_LAUNCHIMAGE_NAME = LaunchImage;

for the app icons during a build which works great. However, if this file is deleted (happened on a project today and took hours to figure out why the icons weren't showing when installing the app), then the app icon is not set during a build and no warnings/info is provided.

This could be a potential time saver for others in the future if the file is not found. I don't know all the implications of that file during xcode builds so I won't pretend to 😄 - it would have saved me several hours tracking down the file was deleted months ago from the project repository and that I had to set the icons in xcode on the build settings page after opening the workspace.

先定位 CLI 中的 iOS 建置處理，並搜尋對 build.xcconfig 或 ASSETCATALOG_COMPILER 設定的參照。在缺少該檔案的情況下重現一次建置，接著判斷警告應出現的位置，並確認在檔案存在時建置不會變更且警告會顯示。