cmGeneratorTarget: Move ExplicitTargetName to VS local generator
cmLocalGenerator: Add ComputeObjectFilenames method.
cmLocalGenerator: Add ComputeObjectFilename method.

The generators need to be able to get the ObjectName for a
single source file.
cmGeneratorTarget: Remove GetObjectName method.

It is no longer used.
cmGeneratorTarget: Compute target objects in LookupObjectLibraries.

Remove the old method from the generators which are now all
functionally identical.

Remove the supporting cmGeneratorTarget::AddObject method.

In a follow-up, the LookupObjectLibraries 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

46019f89 cmGeneratorTarget: Compute target objects in LookupObjectLibraries.
32a6fdb5 cmGeneratorTarget: Remove GetObjectName method.
f94cf628 cmLocalGenerator: Add ComputeObjectFilename method.
a91e614b cmLocalGenerator: Add ComputeObjectFilenames method.
c12b6a60 cmGeneratorTarget: Move ExplicitTargetName to VS local generator