LLVM really dislikes this and will assert, saying something along the
lines of:

```
rustc: llvm/lib/MC/MCSubtargetInfo.cpp:60: void ApplyFeatureFlag(
  llvm::FeatureBitset&, llvm::StringRef, llvm::ArrayRef<llvm::SubtargetFeatureKV>
): Assertion
  `SubtargetFeatures::hasFlag(Feature) && "Feature flags should start with '+' or '-'"`
failed.
```

If they are trying to use features rustc doesn't yet know about,
request a feature request.

Additionally, also warn against using feature names without leading `+`
or `-` signs.

At the very least this serves to deduplicate the diagnostics that are
output about unknown target features provided via CLI.