Unsized locals

[bjorn3]

Implemented in rustc in rust-lang/rust#51131

[bjorn3]

9cfb947 made it possible for functions to accept unsized arguments. Calling them is still not possible, as that requires unsized locals.

[bjorn3]

As per rust-lang/compiler-team#630 unsized locals will be removed from the compiler. No need to implement them in cg_clif any longer.