[Clangd] Support Authority-less URIs

Clangd strips URIs by removing the file:// part but some clients can send file:
which is also valid according to RFC 3896. For example, if a client sends
file:///home/user, it gets converted to /home/user but if a client sends
file:/home/user, it is left untouched and problems arise.

Patch by Marc-Andre Laperle!

Differential Revision: https://reviews.llvm.org/D32234

llvm-svn: 300990