swift234

[ClangImporter] When a submodule is imported, so is the top-level module.

Background: if a Clang module /only/ imports submodules from another module,
we would lose all re-exports from that other module, because we're treating
all decls as part of the top-level module. This actually shows up with UIKit
and QuartzCore due to some weirdness in how the QuartzCore module is set up
(see <rdar://problem/17888079>), resulting in 'CALayer' not being visible
with just 'import UIKit'.

Workaround (this patch): treat Clang imports just like Swift imports: if the
submodule is referenced, so is the top-level module. This isn't great, but
it's not really any more awful than the rest of our Clang modules story.

<rdar://problem/17607060>

Swift SVN r20920