CMake supports the notion of installation components. Right now we have some
custom code for supporting swift components. I think that for installation
purposes, it would be nice to use the CMake component system.
This should be a non-functional change. We should still only be generating
install rules for targets and files in components we want to install, and we
still use the install ninja target to install everything.
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SwiftRemoteMirror.dll out of date constantly as no import library is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.