Bug 1749650 - Ignore clangd index files in gecko root folder. r=firefox-build-system-reviewers,andi

NPOTB DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D135674

.gitignore

@@ -181,3 +181,6 @@ testing/raptor/webext/raptor/auto_gen_test_config.js
 # Ignore ICU4X experimentation data files.
 # See intl/ICU4X.md for more details.
 config/external/icu4x
+
+# Ignore the index files generated by clangd.
+.cache/clangd/index/

.hgignore

@@ -228,3 +228,6 @@ toolkit/components/certviewer/content/package-lock.json
 # Ignore ICU4X experimentation data files.
 # See intl/ICU4X.md for more details.
 ^config/external/icu4x
+
+# Ignore the index files generated by clangd.
+^.cache/clangd/index/