Cut the size of upb_LayoutItem from 12 bytes to 6.

If we require C23 I could do this in the enum declaration.

PiperOrigin-RevId: 700728937

upb/mini_descriptor/decode.c

@@ -49,15 +49,17 @@ typedef enum {
   kUpb_LayoutItemType_Max = kUpb_LayoutItemType_Field,
 } upb_LayoutItemType;
 
-#define kUpb_LayoutItem_IndexSentinel ((uint16_t) - 1)
+#define kUpb_LayoutItem_IndexSentinel ((uint16_t)-1)
 
 typedef struct {
   // Index of the corresponding field.  When this is a oneof field, the field's
   // offset will be the index of the next field in a linked list.
   uint16_t field_index;
   uint16_t offset;
-  upb_FieldRep rep;
-  upb_LayoutItemType type;
+  // These two enums are stored in bytes to avoid trailing padding while
+  // preserving two-byte alignment.
+  uint8_t /* upb_FieldRep*/ rep;
+  uint8_t /* upb_LayoutItemType*/ type;
 } upb_LayoutItem;
 
 typedef struct {