MINOR: [C++] Ignore clangd index files
MINOR: [C++] Ignore clangd index files

clangd 12 has moved index files from .clangd/ to .cache/clangd/.
clangd is a language server for code completion, go-to-definition, etc.

Closes #11700 from cyb70289/ignore-clangd-index

Authored-by: Yibo Cai <e732a3f0de35687e82bda0008a72e443d2f69704@arm.com>
Signed-off-by: Yibo Cai <e732a3f0de35687e82bda0008a72e443d2f69704@arm.com>

apache/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,wesm/arrow,kou/arrow,kou/arrow,laurentgo/arrow,apache/arrow,kou/arrow,laurentgo/arrow,kou/arrow,icexelloss/arrow,laurentgo/arrow,kou/arrow,kou/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,apache/arrow,kou/arrow,kou/arrow,kou/arrow,wesm/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow,icexelloss/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,kou/arrow,apache/arrow,apache/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,laurentgo/arrow,wesm/arrow,laurentgo/arrow,apache/arrow,apache/arrow,laurentgo/arrow,kou/arrow,kou/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow