Allow INTERFACE_SOURCES to specify $<TARGET_OBJECTS> (#14970)

Fix cmTarget::GetSourceFiles to set EvaluateForBuildsystem on the
$<TARGET_PROPERTY:...,INTERFACE_SOURCES> generator expression so that
the $<TARGET_OBJECTS> generator expression is allowed within an
INTERFACE_SOURCES value.

Extend the InterfaceLibrary test to cover this case.  Extend the
RunCMake.TargetObjects test to cover failure of $<TARGET_OBJECTS>
when used through $<TARGET_PROPERTY:...,INTERFACE_SOURCES> in a
non-buildsystem context.
Merge topic 'interface-sources-target-objects' into next

e51f2a7b Allow INTERFACE_SOURCES to specify $<TARGET_OBJECTS> (#14970)