swift234

Allow a local function to capture a local function that itself does not have captures <rdar://problem/18053114>.

When we locked down on local functions capturing other local functions
(<rdar://problem/17862878>), we broke existing (working) code that
only captured local functions that themselves had no captures, which
avoided the SILGen problems that led to <rdar://problem/17862878>.

The very-late checking here is necessary because we compute captures
in a weird order that is not postorder traversal.

Swift SVN r21313