swift234

Iterative type checker: simple circular reference detection.

Put in some rudimentary logic for finding circular references within
the iterative type checker and diagnosing those cycles. The
"rudimentary" part is because we're performing linear searches within
a stack rather than keeping a proper dependency graph, which is
inefficient and could display longer cycles than are actually
present. Additionally, the diagnostic is not specialized to the actual
query, so we get a generic "circular reference" diagnostic. OTOH, we
show all of the declarations involved in the cycle, which at least
lets the user figure out where the cycle occurred.

Enable the iterative type checker for resolving the type of a global
typealiases.

Swift SVN r32572