Prepare to support per-config object library files

Drop object libraries from GetConfigCommonSourceFiles. Call sites such as those in the VS global generator that are used only to reject per-config sources will now allow per-config object library objects. Teach the corresponding generators to deal with per-config object library files and not use GetConfigCommonSourceFiles to get them. Remaining call sites do not need object library files anyway.

This will later allow $<TARGET_OBJECTS:...> generator expressions to evaluate to values that vary by configuration (e.g. because each configuration has its own object files).