Make output of --dump-c --use-system-headers compilable

[peterschrammel]

In order to compile dumped C files with the original header files, we have to

• retain typedefs involving anonymous structs/unions
• prevent the output of structs/unions defined in system headers
• prevent the output of built-ins
• ...

A quick hack is here:
peterschrammel@ff6a7d1

More to come...

[tautschnig]

@peterschrammel Has this been fully addressed by #858?

[tautschnig]

I'm inclined to close this issue as dump-c's output appears to work fine these days. It's certainly not perfect, but #5700 shows that it does generally work. I think we need concrete examples of failures to drive further fixes.