.gitignore: Add .cache and compile_commands.json

.gitignore: Add .cache and compile_commands.json

Enhancing the editor / IDE experience with the use of the language
server protocol (lsp) is getting more and more common. The most widely
used lsp implementation for C/C++ outside of Redmond is clangd.

Clangd uses compile_commands.json, that contains the compiler invocation
for each compilation unit, to get the required $CFLAGS to successfully
located header files etc. This is best automatically generated from with
bear on Makefile based build systems. Hence, there is little value in
adding it to the git repo.

In addition, clangd generates cache files in the .cache folder. Again,
there is no reason in tracking it with git.

Change-Id: Ic5165d10aca3a1cc9e9398af9dd2fbf0977608b3
Signed-off-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/7414
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>