Add a mechanism for hiding, skipping, and modifying the generated op

functions for C++.  A souped-up version of the hidden_ops mechanism in
Python, the intent is to use this for most or all of the client
languages, with a common list of changes to make in a common file and
per-language overrides.

Also:
* include the documentation for outputs in the generated comments
* several updates to C++ API to match Python
* fix C++ shape function for ConcatV2 now that we use it by default
* split op_gen_lib out of core:framework, since it is only used by
  the op generators, and I don't want to add another proto to
  mobile builds
Change: 146267344