[clangd] Don't crash on LSP calls for non-added files

Summary:
We will return errors for non-added files for now.
Another alternative for clangd would be to read non-added files from
disk and provide useful features anyway.

There are still some cases that fail with assertion (e.g., code
complete). We should address those too, but they require more subtle
changes to the code and therefore out of scope of this patch.

Reviewers: sammccall, ioeric, hokein

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42164

llvm-svn: 322637