fix incorrect LLVM IR for union constant when active field is void
fix incorrect LLVM IR for union constant when active field is void

found in the llvm6 branch with llvm assertions on

raulgrell/zig,andrewrk/zig,Dimenus/zig,raulgrell/zig,raulgrell/zig,Dimenus/zig,zig-lang/zig,andrewrk/zig,Dimenus/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig