Support API usage when clangd is disabled or failed to initialize properly

[HighCommander4]

Fixes #721

[HighCommander4]

@thegecko I can't actually add you as a reviewer on this patch, but would appreciate if you could have a look anyways, and see if it looks like a suitable alternative to #727

[HighCommander4]

@thegecko I can't actually add you as a reviewer on this patch

(I guess that's because you're not a project member; and I don't have admin access to the repo to make you a project member. @hokein, perhaps you can help with that?)

[thegecko]

👍 This looks a lot neater than #727, I've made one minor suggestion around a static factory function.

There are still a couple of non-null assertions which I addressed in #727. These could be picked up in a subsequent PR.

• vscode-clangd/src/install.ts

  Line 123 in c7c91e6

  let p = config.get<string>('path')!;

• vscode-clangd/src/clangd-context.ts

  Line 123 in c7c91e6

  let items = (Array.isArray(list) ? list : list!.items).map(item => {

[HighCommander4]

Thanks!

There are still a couple of non-null assertions which I addressed in #727. These could be picked up in a subsequent PR.

* https://github.com/clangd/vscode-clangd/blob/c7c91e644eec4cd46b62770336fac2a268426675/src/install.ts#L123

* https://github.com/clangd/vscode-clangd/blob/c7c91e644eec4cd46b62770336fac2a268426675/src/clangd-context.ts#L123

I went ahead and fixed these.

[thegecko]

Thanks for adding those changes, looks great.