[clangd] Don't run the prepare for tweaks that are disabled.

Summary: Previously, we ran the prepare, even for the tweaks that are disabled.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64565

llvm-svn: 365882