[clangd] Fix Go to Definition not working in VSCode extension

Summary:
The URI conversion logic was returning 'undefined' when going from server to
VSCode which broke the Go to Definition functionality.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D35215

llvm-svn: 307715