Teach cmGlobalGenerator::CheckTargets to include OBJECT_LIBRARY targets
in the check for source file existence.
Extend the RunCMake.ObjectLibrary test to cover this case.
Teach the RunCMake.ObjectLibrary test to verify that languages used in
an OBJECT library are propagated to targets that use it so that the
languages can be included in link analysis.
Add "RunCMake.ObjectLibrary" test to verify that unsupported use cases
are rejected with errors:
* An OBJECT library may not reference another object library
* An OBJECT library may not be referenced in target_link_libraries
* An OBJECT library may not contain non-compiling sources
* An OBJECT library may not have pre/post build/link commands
* An OBJECT library may not be installed, exported, or imported
Also verify that invalid $<TARGET_OBJECTS:...> expressions are
diagnosed.
