Skip the pointless tupling/untupling of argument types in trans_closure

Skip the pointless tupling/untupling of argument types in trans_closure

The tupling only happens for actual closures, same for the untupling.
The only code that actually sees the tupled types is some debugging
output for which it is actually rather confusing to have the types
tupled, because neither the function signature in Rust nor the
function signature for LLVM has them tupled.