[clangd] Show background index status using LSP 3.15 work-done progress notifications

Summary:
It simply shows the completed/total items on the background queue, e.g.
 indexing: 233/1000
The denominator is reset to zero every time the queue goes idle.

The protocol is fairly complicated here (requires creating a remote "progress"
resource before sending updates). We implement the full protocol, but I've added
an extension allowing it to be skipped to reduce the burden on clients - in
particular the lit test takes this shortcut.

The addition of background index progress to DiagnosticConsumer seems ridiculous
at first glance, but I believe that interface is trending in the direction of
"ClangdServer callbacks" anyway. It's due for a rename, but otherwise actually
fits.

Reviewers: kadircet, usaxena95

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D73218