Add glslang as a submodule

This will be used by benchmarks and unit tests.

Bug: b/158655757
Change-Id: Idca9f31be946dc3209b8c93cd2195dd43b1e4233
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45649
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/.gitmodules b/.gitmodules
index d010d48..f876631 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -16,3 +16,6 @@
 [submodule "third_party/benchmark"]
 	path = third_party/benchmark
 	url = https://github.com/google/benchmark.git
+[submodule "third_party/glslang"]
+	path = third_party/glslang
+	url = https://github.com/KhronosGroup/glslang.git
diff --git a/third_party/glslang b/third_party/glslang
new file mode 160000
index 0000000..8111268
--- /dev/null
+++ b/third_party/glslang
@@ -0,0 +1 @@
+Subproject commit 8111268575a41753dfb6f9afc40b7c096af769aa