cmd/cgo: fix line info in _cgo_gotypes.go

cmd/cgo: fix line info in _cgo_gotypes.go

Don't write line info for types, we don't have it.
Otherwise types look like:

type _Ctype_struct_cb struct {
//line :1
      on_test *[0]byte
//line :1
}

Which is not useful. Moreover we never override source info,
so subsequent source code uses the same source info.
Moreover, empty file name makes compile emit no source debug info at all.

Update #17190

Change-Id: I7ae6fa4964520d7665743d340419b787df0b51e8
Reviewed-on: https://go-review.googlesource.com/29713
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>