Fix crash when exporting functions with const enums #2384

[chrischtel]

Right now if you export a function that takes a const enum,
the compiler crashes with "TODO reached" in header generation.

see #2384
minimal repro.:

enum Foo : const int
{
    A,
}

fn void test(Foo f) @export
{

}

This patch makes header_gen_maybe_generate_type fall back to the enum’s
underlying integer type, so header generation works instead of failing.

It’s ABI-correct since const enums lower to their base type anyway.
This just fixes the crash for now; nicer typedef/constant output could
be added later if wanted.

[lerno]

Yeah, better have it working. Thank you.