[clangd] Use new URI with scheme support in place of the existing LSP URI

Summary:
o Replace the existing clangd::URI with a wrapper of FileURI which also
carries a resolved file path.
o s/FileURI/URI/
o Get rid of the URI hack in vscode extension.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42419

llvm-svn: 323660