We were previously treating all the builds as shared, which is not the
case for the host library build of SwiftRemoteMirror. The warnings were
lost in the interminable spew from the build which is now fixed and this
stands out.
Mark the public interfaces with the appropriate visibility/dll storage.
This fixes an issue with the Windows build which keeps the
SwiftRemoteMirror.dll out of date constantly as no import library is
created. That occurs due to the fact that the library does not export
any interfaces.
Take the opportunity to move the public interfaces to protected
visibility on ELF.
