Genex: Only evaluate TARGET_OBJECTS to determine target sources.
The output of this expression may contain macros for IDEs to replace such as $(Configuration), $(CURRENT_ARCH) etc. To avoid generating content which is not usable in other contexts, report an error if there is an attempt to use it in other contexts. This commit may be reverted in the future if a solution to the above difference is implemented.
Tests/RunCMake/TargetObjects/NoTarget-result.txt → Tests/RunCMake/TargetObjects/BadContext-result.txt