[clangd] Suggest adding missing includes for typos (like include-fixer).

Summary:
This adds include-fixer feature into clangd based on D56903. Clangd now captures
diagnostics caused by typos and attach include insertion fixes to potentially
fix the typo.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57021

llvm-svn: 353380