* disallow embedding of C type (Fixes issue 2552)
* detect 0-length array (Fixes issue 2806)
* use typedefs when possible, to avoid attribute((unavailable)) (Fixes issue 2888)
* print Go types constructed from C types using original C types (Fixes issue 2612)

This fix changes _cgo_export.h to repeat the preamble from import "C".
Otherwise the fix to issue 2612 is impossible, since it cannot refer to
types that have not been defined.  If people are using //export and
putting non-header information in the preamble, they will need to
refactor their code.

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/5672080