swift234

Rework the AST representation of CollectionExprs to maintain
a list of their elements, instead of abusing TupleExpr/ParenExpr
to hold them.

This is a more correct representation of what is going on in the
code and produces slightly better diagnostics in obscure cases.

However, the real reason to fix this is that the ParenExpr's that
were being formed were not being installed into the "semantic"
view of the collection expr, not getting type checked correctly,
and led to nonsensical ParenExprs.  These non-sensical ParenExprs
blocked turning on AST verification of other ones.

With this fixed, we can finally add AST verification that 
IdentityExpr's have sensible types.



Swift SVN r27850