[clangd] Use capacity() instead of size() in RefSlab::bytes()

Patch by Nathan Ridge.

Reviewers: gribozavr

Reviewed By: gribozavr

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60040

llvm-svn: 357454