[clangd] Avoid cache main file status in preamble.

Summary: Main file can certainly change when reusing preamble.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D52991

llvm-svn: 344024