[clangd] Add support for different file URI schemas.

Summary: I will replace the existing URI struct in Protocol.h with the new URI and rename FileURI to URI in a followup patch.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D41946

llvm-svn: 323101