Arguments retainer_part_nonreadable: clear implicits.

Lemma join_parts:
forall L R (HC: Share.comp L = R) sh1 sh2 sh,
join sh1 sh2 (Share.glb L sh) ->
Share.glb L sh1 = sh1 /\
Share.glb R sh1 = Share.bot /\
Share.glb L sh2 = sh2 /\
Share.glb R sh2 = Share.bot.