Add new script to update external dependencies for the SPIRV backend. (#5246)

This simplifies the updating of the external dependencies for the SPIRV
backend.

Tested on Linux.

utils/update_spirv_deps.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+if [ ! -d external -o ! -d .git -o ! -d azure-pipelines ] ; then
+  echo "Run this script on the top-level directory of the DXC repository."
+  exit 1
+fi
+
+set -ex
+git submodule foreach 'set -x && git switch main && git pull --ff-only'
+git add external
+exit 0