[CMake][RFC] Prune add_public_tablegen_target for each local tablegen

We have been defining add_public_tablegen_target for long time even if it is used only within the module locally. You can find actual examples in llvm/tools, or simply run fgrep -lr add_public_tablegen_targets.
Does anyone remember why? Just for IDE users to execute tablegen rules locally to reconfirm if it would work?

Generated headers may be specified as SOURCES in add_library/add_executable without add_custom_target. I am certain local add_custom_target would be redundant practically.

I propose pruning them. ⚙ D150828 [CMake] Prune add_public_tablegen_target for local tablegen