swift234

Sema: Fix opened existentials leaking out in diagnostics

getTypeOfIndependentSubExpression() might replace some values in the
given expression with OpaqueValueExprs, but if the type checker
decided to insert the corresponding OpenExistentialExpr at the top
level, the AST would now be in an inconsistent state, since
getTypeOfIndependentSubExpression() does not return a new expression
to the caller.

Ideally we would separate out type checking from expression rewriting
so that the latter is only performed when we know the expression
type checks, but that is a bigger project. For now, erase open
existentials after re-typechecking a sub-expression.

Fixes <rdar://problem/20598568>.

Swift SVN r29400