swift234

Reapply the fix for <rdar://problem/21369926> Malformed Swift Enums crash playground service

Completely revamp getTypeOfIndependentSubExpression, to return the subexpression produced by 
type checking instead of just a type. This is important for cases when type checking changes
the root of the AST (e.g. resolving an unresolved_dot_expr) and allows us to eliminate grungy 
and unsafe recovery code that was in place to work around this.  The last point makes the
examples in Constraints/lvalues.swift better (giving a somewhat generic error instead of an 
specific-but-incorrect error that 'z' is immutable), 

but more importantly, it fixes a class of crashers like:
 <rdar://problem/21369926> Malformed Swift Enums crash playground service 
where we'd end up with a LiteralExpr typed as Int instead of a Builtin integer type of some sort.



Swift SVN r30028