Explicitly enable OBJECT library "target".

With this, one can build a lib from the objects of other libs:
set(SOURCES
  $<TARGET_OBJECTS:obj.clingInterpreter>
  $<TARGET_OBJECTS:obj.clingMetaProcessor>
  $<TARGET_OBJECTS:obj.clingUtils>
  )

Reviewed by Chris Bieneman - thanks!

llvm-svn: 257459