This amends chapuni's r246156 to handle an Xcode quirk, one even called out
in the CMake documentation:
Some native build systems may not like targets that have only object files,
so consider adding at least one real source file to any target that
references $<TARGET_OBJECTS:objlib>.
I've limited the scope of this hack to Xcode for now.
llvm-svn: 247305
Jordan Rose