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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 <yibo.cai@arm.com>
Signed-off-by: Yibo Cai <yibo.cai@arm.com>