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
https://microsoft.github.io/language-server-protocol/implementors/servers/ .

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D26555