[clangd] Add fallbackFlags initialization extension.

Summary:
This allows customizing the flags used when no compile database is
available. It addresses some uses of the old extraFlags extension.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D53688

llvm-svn: 345973