[clangd] Assert path is absolute when assigning to URIForFile.

Summary:
The assertion will point directly to misbehaving code, so that
debugging related problems (like the one fixed by r325029) is easier.

Reviewers: hokein, ioeric, sammccall

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43246

llvm-svn: 325337