remove redundant '-> Void' return types from functions in stdlib source
remove redundant '-> Void' return types from functions in stdlib source

since omitting it appears to be the convention followed elsewhere in
the code base.

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