[clangd] Rebuild dependent files when a header is saved.

Summary:
Caveats:
 - only works when the header is changed in the editor and the editor provides
   the notification
 - we revalidate preambles for all open files (stat all their headers) rather
   than taking advantage of the fact that we know which file changed.
   This is much simpler!

Fixes https://github.com/clangd/clangd/issues/107

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77847