[clangd] Don't invalidate LSP-set compile commands when closing a file.

Summary:
It doesn't make much sense: setting them is not coupled to opening the file,
it's an asynchronous notification.

I don't think this is a breaking change - this behavior is hard to observe!

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D53642

llvm-svn: 345231