[clangd] Don't override the preamble while completing inside it, it doesn't work.

Summary:
To stay fast, enable single-file-mode instead. This is fine since completions
in the preamble are simple.

The net effect for now is to suppress the spurious TopLevel completions when
completing inside the preamble.
Once Sema has include directive completion, this will be more important.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D52071

llvm-svn: 342230