[clangd] Canonicalize file path in URIForFile.

Summary:
File paths in URIForFile can come from index or local AST. Path from
index goes through URI transformation and the final path is resolved by URI
scheme and could be potentially different from the original path. Hence, we
should do the same transformation for all paths. We do this in URIForFile, which
now converts a path to URI and back to a canonicalized path.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D54845

llvm-svn: 347739