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

Some day we'll close the hole for @exported in the previous commit.

Swift SVN r29441

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