$PROJECT/.cache/clangd/index is the intended location for project index
data when using clangd as the language server. Ignore this directory to
keep the git status clean.

Signed-off-by: Wang, Lei <lei4.wang@intel.com>
Message-Id: <20220907150010.2047037-1-lei4.wang@intel.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>