swift234

Break SwiftRemoteMirror into its own target.

We want to be able to build a target with just the platform-specific
libswiftRemoteMirror library. This should be a change in build logic
for existing presets but allows for a separate preset to be defined
that just builds the library if/when it's necessary.

- Add --build-swift-remote-mirror option to build-script-impl
- Add swift-remote-mirror known install component.
- Only add SwiftRemoteMirror targets if SWIFT_BUILD_REMOTE_MIRROR is on.
- Move libswiftRemoteMirror into the swift-remote-mirror install component.
- Add swift-remote-mirror install components to existing presets.

rdar://problem/27085666