[clangd] Add fixes for clang "include <foo.h>" diagnostics

Clang doesn't offer these fixes I guess for a couple of reasons:
 - where to insert includes is a formatting concern, and clang shouldn't
   depend on clang-format
 - the way clang prints diagnostics, we'd show a bunch of basically irrelevant
   context of "this is where we'd want to insert the include"

Maybe it's possible to hack around 1, but 2 is still a concern.
Meanwhile, bolting this onto include-fixer gets the job done.

Fixes https://github.com/clangd/clangd/issues/355
Fixes https://github.com/clangd/clangd/issues/937

Differential Revision: https://reviews.llvm.org/D114667