swift234

Rework ReplaceDependentTypes to call into applyUnboundGenericArguments
to remap an unbound generic type into a bound generic type.  This shares
a common codepath to make way for future progress.

This exposed a case where we'd produce invalid ASTs in some testcases
in the validation tests because started asserting on the invalid code.
Solve this by detecting the problem and producing a circularity error.

Overall, NFC.