[clangd] Debounce rebuilds responsively to rebuild times.

Summary:
Old: 500ms always. New: rebuild time, up to 500ms.

Fixes https://github.com/clangd/clangd/issues/275

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73949