Add dummy definition for referenced but never-defined structs

[lucic71]

This handles opaque pointers to user defined forward declared structs. It's allowed to use such pointers, even though the struct is not defined in any translation unit.

To implement this, I modified record_decls_ to also keep an extra bool with the following meaning:

    // record name -> true if a definition has been emitted, false if only
    // referenced.
    std::unordered_map<std::string, bool> entries_;

At the end of visiting all TU's, for the record_decls_ that are only referenced (not defined), a dummy struct definition is generated:

#[repr(C)]
pub struct opaque {
    _opaque: [u8; 0],
}

[lucic71]

pub struct opaque is enough, it doesn't have to be

#[repr(C)]
pub struct opaque {
    _opaque: [u8; 0],
}