Xcode: Fix object library references in multi-project trees (#13452)
In cmGlobalXCodeGenerator::Generate we generate a .xcodeproj for each directory in the tree containing a project() command. First we iteratively use SetGenerationRoot to add "ALL_BUILD" and other targets to each project. This leaves "CurrentProject" set to the last project when we invoke cmGlobalGenerator::Generate, which is not the same as the top-level project if any subdirectories invoke the project() command. When cmGlobalGenerator::Generate reaches CreateGeneratorTargets it constructs cmGeneratorTarget and calls ComputeTargetObjects exactly once per target. In this context the value of CurrentProject is undefined so we cannot pass it to GetObjectsNormalDirectory. Use "$(PROJECT_NAME)" instead so it will adapt automatically to each project. Also teach Tests/ObjectLibrary to cover this case.