[clangd] Add add commit characters to the server capabilities

Summary:
Make it more convinient for the clients to select completion items by
providing a set of default characters (punctuation).

Related issue: https://github.com/clangd/clangd/issues/284

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74606