gitignore: ignore clangd files and directories

gitignore: ignore clangd files and directories

In order to use clangd it is currently necessary to add some
configuration into a .clangd file. Unfortunately, this is very situation
specific and cannot meaningfully be upstreamed. (For example, I need to
manually specify "-std=c++17" to work around the issue that gcc defaults
to c++17 and clang defaults to c++14.)

While at it also ignore the .cache directory where clangd stores its
index.