Update documentation regarding updating the clang toolchain

Add a reference to the libclang package to the list of packages
which should be uploaded from the staging bucket to the production
bucket when the clang toolchain is rolled.

See this commit for details on the new libclang package:
http://crrev.com/c/1874450

Change-Id: I2f0264e95a5fad4dc370647d60f894c30d26fd89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877605Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Taylor Woll <Taylor.Woll@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#709106}

docs/updating_clang.md

@@ -30,6 +30,8 @@ An archive of all packages built so far is at https://is.gd/chromeclang
             gs://chromium-browser-clang/$x/translation_unit-$rev.tgz ; \
         gsutil.py cp -n -a public-read gs://chromium-browser-clang-staging/$x/llvm-code-coverage-$rev.tgz \
             gs://chromium-browser-clang/$x/llvm-code-coverage-$rev.tgz ; \
+        gsutil.py cp -n -a public-read gs://chromium-browser-clang-staging/$x/libclang-$rev.tgz \
+            gs://chromium-browser-clang/$x/libclang-$rev.tgz ; \
         done && gsutil.py cp -n -a public-read gs://chromium-browser-clang-staging/Mac/lld-$rev.tgz \
             gs://chromium-browser-clang/Mac/lld-$rev.tgz
     ```