Input C/C++ Header

enum long_prefix_to_remove_foo {
  BAR = 0
};

int long_prefix_to_remove_bar();

There is no way to implement ParseCallbacks::item_name that can follow different naming logic for these two items.

I propose that item_name be refactored to look more like generated_name_override, which passes in an info struct instead of solely a name.

Cc @vsukhoml