Add compile_commands.json to gitignore

This updates .gitignore to ignore files related to clangd language
server.

Added the following to .gitignore:
* compile_commands.json: Generated by ninja or other build tools to be
  used by C/C++ clangd.
* .cache: Cache folder of clangd

Bug: 176837458
Tag: #floss
Test: Manual - Use clangd with vim

Change-Id: Ic25fc5670ae67ca920402a7a918d8e1eb9a9f3c2