[mlir][llvm] Add llvm.target_features features attribute (#71510)

This patch adds a target_features (TargetFeaturesAttr) to the LLVM
dialect to allow setting and querying the features in use on a function.

The motivation for this comes from the Arm SME dialect where we would
like a convenient way to check what variants of an operation are
available based on the CPU features.

Intended usage:

The target_features attribute is populated manually or by a pass:

```mlir
func.func @example() attributes {
   target_features = #llvm.target_features<["+sme", "+sve", "+sme-f64f64"]>
} {
 // ...
}
```

Then within a later rewrite the attribute can be checked, and used to
make lowering decisions.

```c++
// Finds the "target_features" attribute on the parent
// FunctionOpInterface.
auto targetFeatures = LLVM::TargetFeaturesAttr::featuresAt(op);

// Check a feature.
// Returns false if targetFeatures is null or the feature is not in
// the list.
if (!targetFeatures.contains("+sme-f64f64"))
    return failure();
```

For now, this is rather simple just checks if the exact feature is in
the list, though it could be possible to extend with implied features
using information from LLVM.