cpp.Int64 -> haxe.Int64 silently converts to Int32

[haxiomic]

Ran into this while trying to use haxe.Int64 to get the low and high bits of a cpp.Int64:

The following

var hx64: haxe.Int64 = (cpp64: cpp.Int64);

Generates

cpp::Int64Struct hx64 = ( ::cpp::Int64Struct(( (int)(cpp64) )))

Where the unexpected (int) cast means we lose the top 32 bits. This is because cpp.Int64 can auto-cast to Int and the haxe.Int64 abstract has auto-cast from Int but not Int64. A fix is to add an auto-cast from cpp.Int64

PR + regression test on the way