Add clangd cache to ignore (#13856)

While standard library is cached to a user's directory such as
$XDG_CACHE_HOME, directory-specific clangd indexes are stored to .cache
subdirectory.
https://github.com/llvm/clangd-www/blob/main/design/indexing.md#backgroundindex

.gitignore

@@ -12,6 +12,7 @@ compile_commands.json
 /.deps/
 /tmp/
 /.clangd/
+/.cache/clangd/
 
 .DS_Store
 *.mo