Start stubbing out more parts of Type::join().

Improve support for Optional among other things.

Return Any when it is really the best answer given the types involved,
or nullptr if we cannot yet produce an accurate result.

This implementation returns Any? when joining Any with an
Optional<T>. In our type system both Any and Any? are effectively
subtypes of one another since each can hold all the values that the
other can hold. So this choice is somewhat arbitrary, but does line up
nicely with the notion that T is always a subtype of T?.

Author: rudkx

include/swift/AST/Type.h

@@ -368,15 +368,19 @@ class Type {
   /// class D { }
   /// \endcode
   ///
-  /// The join of B and C is A, the join of A and B is A. However, there is no
-  /// join of D and A (or D and B, or D and C) because there is no common
-  /// superclass. One would have to jump to an existential (e.g., \c AnyObject)
-  /// to find a common type.
+  /// The join of B and C is A, the join of A and B is A.
+  ///
+  /// The Any type is considered the common supertype by default when no
+  /// closer common supertype exists.
+  ///
+  /// In unsupported cases where we cannot yet compute an accurate join,
+  /// we return None.
   ///
   /// \returns the join of the two types, if there is a concrete type
   /// that can express the join, or Any if the only join would be a
-  /// more-general existential type
-  static Type join(Type first, Type second);
+  /// more-general existential type, or None if we cannot yet compute a
+  /// correct join but one better than Any may exist.
+  static Optional<Type> join(Type first, Type second);
 
 private:
   // Direct comparison is disabled for types, because they may not be canonical.