[clangd] Don't expose vfs in TUScheduler::runWithPreamble.

Summary:
It was previously an easy way to concurrently access a mutable vfs,
which is a recipe for disaster.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, cfe-commits, ioeric

Differential Revision: https://reviews.llvm.org/D44463

llvm-svn: 327537