Revert "Switch overlays from using @exported to -import-underlying-module."
Revert "Switch overlays from using @exported to -import-underlying-module."

This reverts r29441 because it breaks the Linux build. I'll talk to Dmitri
about this tomorrow.

See rdar://problem/21254367

Swift SVN r29444

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation