Add third_party/mlir-hlo (#4340)

diff --git a/.gitmodules b/.gitmodules
index f5306b8..f363c60 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -53,3 +53,6 @@
 [submodule "third_party/liburing"]
 	path = third_party/liburing
 	url = https://github.com/axboe/liburing.git
+[submodule "third_party/mlir-hlo"]
+	path = third_party/mlir-hlo
+	url = https://github.com/tensorflow/mlir-hlo.git