Support for AffineMapAttr.

PiperOrigin-RevId: 205157390
diff --git a/lib/Parser/Parser.cpp b/lib/Parser/Parser.cpp
index 3d5c908..d220787 100644
--- a/lib/Parser/Parser.cpp
+++ b/lib/Parser/Parser.cpp
@@ -598,6 +598,11 @@
     return builder.getArrayAttr(elements);
   }
   default:
+    // Try to parse affine map reference.
+    auto* affineMap = parseAffineMapReference();
+    if (affineMap != nullptr)
+      return builder.getAffineMapAttr(affineMap);
+
     // TODO: Handle floating point.
     return (emitError("expected constant attribute value"), nullptr);
   }