[clangd] Mechanism to make update debounce responsive to rebuild speed.

Summary:
Currently we delay AST rebuilds by 500ms after each edit, to wait for
further edits. This is a win if a rebuild takes 5s, and a loss if it
takes 50ms.

This patch sets debouncepolicy = clamp(min, ratio * rebuild_time, max).
However it sets min = max = 500ms so there's no policy change or actual
customizability - will do that in a separate patch.

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

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73873