[objcmt] When whitelisting the headers we want to modify, allow changing the
the ObjC implementation declarations, just don't change implementations for
classes that are not in the whitelisted headers.

For example, if we change a method to return 'instancetype' we should also
update the method definition in the implementation.
[objcmt] When emitting a remap file, use a json format with the edit entries, instead of applying the changes
to a temp file directly.

This allows to combine the edits when they can be different based on whether you saw
the implementation or not, e.g. with the designated initializer migration.