[FIRRTL] Add ArrayRefAttr to FIRRTL

A number of ArrayAttr inherent attributes in the FIRRTL dialect have custom
accessors defined, because we want the getter to return an ArrayRef<Attribute>,
not an ArrayAttr.  This PR adds an untyped ArrayRefAttr that will generate the
accessors we want. Maybe someday this could be delivered upstream to MLIR, if
it proves useful to us.

include/circt/Types.td

@@ -26,4 +26,14 @@ def APSIntAttr : Attr<CPred<"$_self.isa<::mlir::IntegerAttr>()">,
   let convertFromStorage = "$_self.getAPSInt()";
 }
 
+// Compared to the standard ArrayAttr, the accessor to an ArrayRefAttr will
+// convert the attribute to an ArrayRef, while an ArrayAttr accessor just
+// returns itself.
+def ArrayRefAttr :
+  ArrayAttrBase<
+    CPred<"::llvm::isa<::mlir::ArrayAttr>($_self)">, "array attribute"> {
+  let returnType = [{ ::llvm::ArrayRef<Attribute> }];
+  let convertFromStorage = [{ $_self.getValue() }];
+}
+
 #endif // CIRCT_TYPES