swift234

Sema: Don't set type of TypeAliasDecl until we resolve the alias type

This changes the behavior to match NominalTypeDecls, which don't have a type
until everything is set up either. In a few places we construct TypeAliasDecls
from known types directly, and we have to call computeType().

Fixes <rdar://problem/19534837>.

Swift SVN r30386