Summary: [Clangd] Added hidden command line option -tweaks to specify which tweaks to enable

- Only for development purposes
- Disabled tweaks in fixits-duplications test
Reviewers: sammccall, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63989

llvm-svn: 364809