The LSP progress handler would put non-progress messages (such as from
clangd or pyls; not part of the LSP spec) directly into
`client.messages`, while `vim.lsp.util.get_progress_messages()` would
try to fetch them from `client.messages.messages` instead (and come up
empty everytime). This would result in these messages never being
cleaned up by `get_progress_messages()`.
This commit fixes that by treating those messages like show-once
progress messages (by setting `done=true` immediately).
7e7fdca163
