swift234

Revise the parser and AST representation of #available to be part of StmtCondition
instead of being an expression.

To the user, this has a couple of behavior changes, stemming from its non-expression-likeness.
 - #available cannot be parenthesized anymore
 - #available is in its own clause, not used in a 'where' clause of if/let.

Also, the implementation in the compiler is simpler and fits the model better.  This
fixes:
<rdar://problem/20904820> Following a "let" condition with #available is incorrectly rejected



Swift SVN r28521