[NFC] Let Git ignore .cache/clangd directory (#2542)

diff --git a/.gitignore b/.gitignore
index cb9d239..7db7ef9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -37,4 +37,5 @@
 
 # Source indexing files
 compile_commands.json
+.cache/clangd
 .clangd/