[gitignore] Add ignore for /.clangd file

A top level .clangd file can be used to configure clangd for the project. Git should ignore it:
https://clangd.llvm.org/config.html

Change-Id: I40ce045bd3058dda2bee1d35bf5bff9147545760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2694410
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#854681}

.gitignore

@@ -60,6 +60,7 @@ tags
 Thumbs.db
 v8.log
 vs-chromium-project.txt
+/.clangd
 /.clangd/
 /.clangd-index/
 # Settings directories for eclipse