Support undefined typedef union

[rschwarz]

This is an attempt to fix #214.

Unfortunately, the tests did not pass for me, so I did not add any for this case yet.

[rschwarz]

FYI, this is what fails for me:

function: Test Failed at /home/rs/.julia/dev/Clang/test/functions.jl:17
  Expression: defs == (nothing, -10)
   Evaluated: (Any[],) == (nothing, -10)

This is on Clang master with Julia 1.0.2.

[rschwarz]

Right now, typedef union uUnion UUNION will generate this:

struct uUnion
    _uUnion::Cvoid
end

const UUNION = Cvoid

Here, uUnion is not correct, but at least UUNION is there.

[Gnimuc]

Fixed in #210, the result should be:

const uUnion = Cvoid
const UUNION = uUnion

[Gnimuc]

closed in favor of #216