backend=auto (now the default if backend is omitted) means to let the compiler
pick whatever backend it wants as the default. This is important for platforms
where we don't yet have a self-hosted backend, such as loongarch64.
Also purge a bunch of redundant target=native.
This logic was previously in Sema, which was unnecessary complexity, and meant the issue was not detected unless the declaration was semantically analyzed. This commit finishes the work which 941090d started.
Resolves: #17916
