GeezerVison

When the following conditions are satisfied,
`isSimpleTypeRelatedTo(source, target)` returns true, which is false
positive.

 - `strictNullChecks` is disabled

 - `source` is `undefined`

 - `target` is an empty intersection type which reduces to `never` by
   discriminants

In this case, `isSimpleTypeRelatedTo()` should actually return `false`,
which does *not* mean `source` doesn't relate to `target`, but rather
tells the checker to further investigate their relation.

When types are checked if they are `never` by their `TypeFlags`, they
must first be reduced, because intersection types of object-like types
which reduce to `never` don't have `TypeFlags.Never`. See microsoft#36696 for
details.

When a function is declared to return such type and it doesn't contain
any return statement or it only contains return statements without
expression, the error messages are incorrect as the checker sees its
return type as non-`never`.

This commit takes into account that intersection types potentially
reduces to `never` and improves error messages.