[clangd] Add an option in the code to not display number of fixes

Summary:
Only to the APIs, which are used by our embedders.
We do not plan to add a user-facing option for this.

Reviewers: sammccall, ioeric

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58387

llvm-svn: 354349