[cleanup] Add .cache to .gitignore

Recent versions of clangd put lots of stuff into .cache,
cluttering `git status` output.

No-Try: true
Change-Id: I0b5d78a8b2813bd11ad1f0d32bc8ea314103fe19
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593255Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71780}

.gitignore

@@ -23,6 +23,7 @@
 *~
 .#*
 .*.sw?
+.cache
 .ccls-cache
 .clangd
 .cpplint-cache