issue 4386: lsp-completion-at-point short-circuits the completion…

[clarendonlabs]

… function chain

lsp-completion-mode will make a buffer-local version of the `completion-at-point-functions' list and will add `lsp-completion-at-point' to the head of the list if that symbol is not already present.

However, an `lsp-unconfigure-hook' is called which results in removal of `lsp-completion-at-point' from the buffer-local version. The next time `lsp-completion-mode' runs, `lsp-completion-at-point' is added to the head of the `completion-at-points-functions'.

To fix, a flag variable is set whose use prevents `lsp-completion-at-point' from removed from `completion-at-point-functions' if it was present at the start.

[clarendonlabs]

Following an email with Kien Nguyen, I’ve generated a PR for a possible fix to the short-circuiting issue#4386 reported earler.