swift234

add a specialized diagnostic for variables with no initializer in the REPL. 
We now get:

(swift) var z : Int
<REPL Input>:1:1: error: variables currently must have an initial value when entered at the top level of the REPL
var z : Int
^

instead of a confusing definite init error.  Something better is tracked in
rdar://15157729



Swift SVN r8938