[clangd] Append "(fix available)" to diagnostic message when fixes are present.

Summary:
This would make diagnostic fixits more discoverable, especially for
plugins like YCM.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D57509

llvm-svn: 352764