internal/lsp: reply to shutdown request
internal/lsp: reply to shutdown request

Fixes golang/go#31333

Change-Id: I5cb40baee49c4b1644df5612c977ab1d71942022
Reviewed-on: https://go-review.googlesource.com/c/tools/+/172639
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools