[clangd] Send EOF before resetting diagnostics consumer

Summary:
Some clang-tidy checkers, e.g. llvm-include-order can emit diagnostics
at this callback (as mentioned in the comments).

Clangd was resetting diag consumer to IgnoreDiags before sending EOF, hence we
were unable to emit diagnostics for such checkers.

This patch changes the order of that reset and preprocosser event to make sure
we emit that diag.

Fixes https://github.com/clangd/clangd/issues/314.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83178