cmGlobalGenerator: Make ComputeTargetObjects non-virtual

Implement it in terms of the ComputeObjectFilenames virtual method
on the local generators.

Remove the reimplementation from the global generators which are
now all functionally identical.
cmGeneratorTarget: Compute target objects on demand

Add a ComputeTargetObjects method to compute the object
names.  It takes mapping to populate as an out-parameter so
that it can be extended in the future with parameters
relevant to generator expression evaluation.

Remove the supporting cmGeneratorTarget::AddObject method. It is
no longer needed as the container member is populated directly.

In a follow-up, the UseObjectLibraries usage may be replaced by a
true generator expression evaluator for TARGET_OBJECTS. That
will require generators to use cmGeneratorTarget::GetExternalObjects
which is not currently the case for Xcode and VS generators.
Merge topic 'target-objects-refactor' into next

e8fe5bea cmGeneratorTarget: Compute target objects on demand
6c9dd0ec cmGlobalGenerator: Make ComputeTargetObjects non-virtual
Early return.
Merge topic 'target-objects-refactor' into next

b780f71c Early return.