[clangd] Add a test URI scheme for lit tests to unbreak platform-specific URI failures.

Summary:
This should also fix the current windows buildbot breakage
(http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015/builds/9838/steps/ninja%20check%201/logs/stdio)

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D42735

llvm-svn: 323885