Detect functions that never return (and @noreturn annotation)

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:

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.