clangd: Set a diagnostic on a code action resulting from a tweak

... if there is a match.
This is needed to that clients can can make a connection between a
diagnostic and an associated quickfix-tweak.
Ideally, quickfix-kind tweak code actions would be provided inline along
with the non-tweak fixes, but this doesn't seem easily achievable.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D118976