This support was actually added by removing a superfluous check that was
too eager in cmLinkItemGraphVisitor.cxx.
Extended the existing Graphviz tests to include this particular case,
and re-generated the expected output files.
Fixes: #20720
* Added a fairly comprehensive test suite
* Separated the graph traversal logic from the Graphviz generation
code by introducing a new class, cmLinkItemsGraphVisitor{.h,cxx}
* Made the graph traversal logic less ad-hoc by using existing
methods in the GlobalGenerator; this fixed a few bugs
* Added support for new target types: custom targets, object
and unknown libraries
* Improved support for ALIAS libraries by showing the alias(es)
in the graph
* Introduced new flags to control those new libraries (consistent
with existing flags)
* Updated the documentation
* Removed useless setting to set graph type in dot file
* Improved the node/edge shapes (nicer, more consistent)
* Added a legend to the graph
* Some refactoring and cleanup of the Graphviz generation code
* Added test and fix for issue 19746
