* ZIR encoding for function instructions have a body for the return
   type. This lets Sema for generic functions do the same thing it does
   for parameters, handling `error.GenericPoison` in the evaluation of
   the return type by marking the function as generic.

 * Sema: fix missing block around the new Decl arena finalization. This
   led to a memory corruption.

 * Added some floating point support to the LLVM backend but didn't get
   far enough to pass any new tests.