stage2: return type expressions of generic functions

* 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.

src/Module.zig

@@ -842,6 +842,9 @@ pub const Fn = struct {
 
     pub fn getInferredErrorSet(func: *Fn) ?*std.StringHashMapUnmanaged(void) {
         const ret_ty = func.owner_decl.ty.fnReturnType();
+        if (ret_ty.tag() == .generic_poison) {
+            return null;
+        }
         if (ret_ty.zigTypeTag() == .ErrorUnion) {
             if (ret_ty.errorUnionSet().castTag(.error_set_inferred)) |payload| {
                 return &payload.data.map;