swift234

[Type checker]: Implement 'join' for optional types and a special case for 'nil'.

The join operation for optional types is straightforward to define/implement:

  join(T?, U)  ::= join(T, U)?
  join(T, U?)  ::= join(T, U)?
  join(T?, U?) ::= join(T, U)?

As a special case in the constraint solver, handle the join of a 'nil'
literal with a non-ExpressibleByNilLiteral-conforming concrete type
'T' to produce 'T?'. This allows us, e.g., infer [String?] for the
expressions

  ["hello", nil]

and

   true ? "hello" nil

for example. Fixes rdar://problem/16326914.