[clangd] Dont provide locations for non-existent files.

Summary:
We were getting assertion errors when we had bad file names, instead we
should skip those.

Reviewers: hokein

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

Differential Revision: https://reviews.llvm.org/D55275

llvm-svn: 348359