swift234

[autolinking] Hack: autolink any module containing external definitions.

We're still synthesizing external definitions too eagerly, and things
getting pulled in only through submodules aren't getting properly autolinked.
(That is, AppKit imports <QuartzCore/CIImage.h> but doesn't think that
QuartzCore is visible, because it isn't.)

The first right answer is to detect that a part of QuartzCore is visible even
though the whole thing isn't. The second right answer is to properly handle
Clang submodules as real modules---there are a lot of rough edges there.
<rdar://problem/13140302>. The third right answer is that we shouldn't even
emit references to these symbols until the /user/ needs them.

And what I'm doing now is just to attempt to link to every module that we've
synthesized thunks for.

<rdar://problem/15754311>

Swift SVN r12031