swift234

now that Beta 5 has branched, we can remove some migration stuff from
what will become beta 6.  This resolves:

<rdar://problem/17662823> Beta 6: remove "operator infix" -> "infix operator" rewrite
<rdar://problem/17670579> Remove @assignment fixit upgrade logic
<rdar://problem/17747208> remove auto_closure upgrade logic after Beta 5 branches



Swift SVN r20629