gitignore: Add .clangd and .ccls-cache
Add residues of clangd and ccls to prevent them from being accidentally
included in a commit.
clangd and ccls are two C/C++ Language Server implementations listed in
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D26555