Make sure the add_ commands can only handle appropriate target types.
Document that an ALIAS may not be installed or exported.
cmTarget: Add the ALIASED_TARGET target property

If the target being read is an alias name, this returns the name
of the aliased target.
Test that add_custom_command with an ALIAS works.
Make sure add_custom_target works with an ALIAS.
Test that add_test works with an ALIAS.
Test that $<TARGET_FILE> works.
Test that an OBJECT_LIBRARY ALIAS works.
Test the behavior of $<TARGET_PROPERTY:Alias,NAME>

The result is the name of the aliased target.
Merge topic 'ALIAS-targets' into next

b5673a7 Test the behavior of $<TARGET_PROPERTY:Alias,NAME>
1e920b6 Test that an OBJECT_LIBRARY ALIAS works.
775e9ff Test that $<TARGET_FILE> works.
80a10f0 Test that add_test works with an ALIAS.
556233f Make sure add_custom_target works with an ALIAS.
17c80f0 Test that add_custom_command with an ALIAS works.
a18bb59 cmTarget: Add the ALIASED_TARGET target property
c5802c8 Document that an ALIAS may not be installed or exported.
e9f8f65 Make sure the add_ commands can only handle appropriate target types.