Validation errors when assigning from built in `as void` function/method

[markwpearce]

Built in Brightscript functions that are as void do not return invalid like developer created functions.

When you call built-in as void functions that are on the native roku objects, they return uninitialized. When you call developer-written as void functions, they return invalid.
So the runtime behavior is different even though you'd expect as void to behave the same regardless of where they're defined.
And since uninitialized causes runtime crashes when you try to read them and have slightly different rules than invalid, it's important to know when you're using uninitialized vs invalid.

To fix in V1 -- we could just make the doc scraper build a correct TypedFunctionType that returns UninitializedType when the return is as void
And then validate against uninitialized RHS of assignment statements.

[TwitchBronBron]

Interestingly enough, the assignment itself is not the error. It's when you try to use it that it crashes. Like this:

thing1 = noop()
'this code is fine
if thing1 = invalid then
  print "thing1 is invalid"
end if

thing2 = arr.Append([])
'this code will crash
if thing2 = invalid then
  print "thing2 is invalid"
end if