To reproduce:

1. Download attached "ReproduceOptimizationBug.zip" project and open in Unity
2. Open Build Settings window
3. Make sure that Platform is set to iOS
4. Press "Build and Run" button

Expected result: Unity Editor successfully Builds project
Actual result: Build process fails and throws Errors in Console:

"Exception: Calling TargetGuidByName with name=‘Unity-iPhone’ is deprecated. There are two targets now, call GetUnityMainTargetGuid() - for app or GetUnityFrameworkTargetGuid() - for source/plugins to get Guid instead.
UnityEditor.iOS.Xcode.PBXProject.TargetGuidByName (System.String name) (at /Users/builduser/buildslave/unity/build/External/XcodeAPI/Xcode/PBXProject.cs:170)
UnityEditorInternal.Advertisements.UnityAdsPostprocessor.EditXcodeProject (System.String buildFolder) (at /home/builduser/buildslave/com.unity.ads/package/Source/UnityAdsEditor/UnityAdsPostprocessor.cs:101)
UnityEditorInternal.Advertisements.UnityAdsPostprocessor.PostProcessBuild_iOS (UnityEditor.BuildTarget target, System.String path) (at /home/builduser/buildslave/com.unity.ads/package/Source/UnityAdsEditor/UnityAdsPostprocessor.cs:92)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <a8ed250850854b439cedc18931a314fe>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <a8ed250850854b439cedc18931a314fe>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <a8ed250850854b439cedc18931a314fe>:0)
UnityEditor.Build.BuildPipelineInterfaces+AttributeCallbackWrapper.OnPostprocessBuild (UnityEditor.Build.Reporting.BuildReport report) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/BuildPipelineInterfaces.cs:158)
UnityEditor.Build.BuildPipelineInterfaces+<>c__DisplayClass17_0.<OnBuildPostProcess>b__1 (UnityEditor.Build.IPostprocessBuildWithReport bpp) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/BuildPipelineInterfaces.cs:398)
UnityEditor.Build.BuildPipelineInterfaces.InvokeCallbackInterfacesPair[T1,T2] (System.Collections.Generic.List`1[T] oneInterfaces, System.Action`1[T] invocationOne, System.Collections.Generic.List`1[T] twoInterfaces, System.Action`1[T] invocationTwo, System.Boolean exitOnFailure) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/BuildPipelineInterfaces.cs:356)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)"

"Error building Player: Exception: Calling TargetGuidByName with name=‘Unity-iPhone’ is deprecated. There are two targets now, call GetUnityMainTargetGuid() - for app or GetUnityFrameworkTargetGuid() - for source/plugins to get Guid instead."

"Build completed with a result of ‘Failed’
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)"

"UnityEditor.BuildPlayerWindow+BuildMethodException: 2 errors
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (UnityEditor.BuildPlayerOptions options) [0x00269] in /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPlayerWindowBuildMethods.cs:191
at UnityEditor.BuildPlayerWindow.CallBuildMethods (System.Boolean askForBuildLocation, UnityEditor.BuildOptions defaultBuildOptions) [0x00080] in /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPlayerWindowBuildMethods.cs:95
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)"

Notes:
- This issue seems to happen only on the iOS platform

Reproduced on Unity 2019.3.0a2 and 2019.3.0a9
Not reproduced on Unity 2019.3.0a1
Regression on Unity 2019.3.0a2