internal/lsp: check for context cancellation before showing messages
internal/lsp: check for context cancellation before showing messages

Noticed this as part of investigating golang/go#40567.

Change-Id: I977b1b0a6ceb139e35e087c16b6ab88d66af69a9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/248400
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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