Swift CI tests backward deployment on different host OSes, so this should be covered in
testing. The explicit target doesn't work well with `executable_test` since it leads to
binaries with too-new minimum deployment targets getting executed on older host OSes.
The RemoteMirror library in shipping versions of macOS/iOS/tvOS/watchOS crashes if the compiler
emits a BuiltinTypeDescriptor with size zero. Although this is fixed in top-of-tree RemoteMirror,
we want binaries built with the new compiler to still be inspectable when run on older OSes.
Generate the metadata as an empty struct with no fields when deploying back to these older
platforms, which should be functionally equivalent for most purposes.
Fixes rdar://problem/57924984.
