stage2: fix AstGen for some struct syntaxes

* AstGen: fix not emitting `struct_init_empty` when an explicit type is
   present in struct initialization syntax.
 * AstGen: these two syntaxes now lower to identical ZIR:
   - `var a = A{ .b = c };`
   - `var a = @as(A, .{ .b = c });`
 * Zir: clarify `auto_enum_tag` in the doc comments.
 * LLVM Backend: fix lowering of function return types when the type has
   0 bits.

src/codegen/llvm.zig

@@ -563,8 +563,13 @@ pub const DeclGen = struct {
             }
         }
 
+        const llvm_ret_ty = if (!return_type.hasCodeGenBits())
+            self.context.voidType()
+        else
+            try self.llvmType(return_type);
+
         const fn_type = llvm.functionType(
-            try self.llvmType(return_type),
+            llvm_ret_ty,
             llvm_param_buffer.ptr,
             llvm_params_len,
             .False,