Allow generating hidden visibility functions

[chertov]

• add a generate_hidden_functions option so bindgen can keep functions that Clang marks with hidden/protected visibility (fixes The compiled code has no functions #2989)
• allow Function::parse to honor the new option instead of dropping those declarations unconditionally

[ivmaykov]

I think you need to add some test cases

[emilio]

Seems fine, but let's use the easier-to-manage test framework since there's nothing special about this test. Just use // bindgen-flags: --generate-hidden-functions in the header.

[emilio]

Please reuse the existing test framework instead. I.e. add a header to bindgen-tests/tests/headers and the generated file to bindgen-tests/tests/expectations. That way we can test the whole content.