Detect functions that never return (and @noreturn annotation)

Nobody has claimed this yet.

Some functions, such as `error`, never return control to the caller. This can be used to prove that a value is not nil, for example:

```lua
function my_function(input)
local value = input.value or error('Missing value')
-- The following line would currently raise a "Needs nil check" warning
-- However, if it was nil at runtime, an error would have been raised and it would never get this far.
return value:sub(1, 3)
end
```

It's not just `error` that can cause this, however. A custom function could be written that never returns as well (the most obvious way is that the function unconditionally calls `error`, but there may be other ways involving coroutines or functions implemented in C). A `@noreturn` annotation could be used to indicate this, similar to `@nodiscard`.

No file or test is named in the issue. Start by tracing the nil-check warning and the existing @nodiscard annotation handling, then determine how non-returning built-ins, custom functions, and @noreturn should be represented; done means the example no longer warns and the annotation behavior is covered.

Written by the indexing model from the issue text.