Ignore clangd-generated files

Absent this change, using clangd / YouCompleteMe with a PDFium checkout
will cause git to report `.cache` and `compile_commands.json` as
untracked files. These are never useful to track / submit, so we should
ignore them.

Change-Id: I27a3b0661776e5ee96ce1b3023a38fa49b8d8453
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/96290
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

.gitignore

diff --git a/.gitignore b/.gitignore
index 899c841..f94b732 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 /base
 /build
 /buildtools
+/compile_commands.json
 /out
 /testing/corpus
 /third_party/abseil-cpp
@@ -29,6 +30,8 @@
 /tools/skia_goldctl
 /v8
 /xcodebuild
+# Generated by clangd.
+/.cache/
 .vscode/
 .vscode-server/
 .DS_Store