[clang] Add `forceReload` clangd extension to 'textDocument/didChange'

Summary:
- This option forces a preamble rebuild to handle the odd case
  of a missing header file being added

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73916