swift234

Sema: Improve a circularity-breaking hack

The old check was problematic because it would sometimes return
ErrorType from accessing the generic parameters of a type that was
in the process of having its generic signature validated.

Instead fix the root cause of the circularity, which was recursion
through resolveType() when resolving a type's inheritance clause.

To fix it, let's just not walk adopted protocols when looking up
the superclass of a class. Inheriting from an associated type does
not seem very useful, and will probably be hard to get right in
the general case.

I wasn't able to reduce a useful test case, but this patch prevents
compiler_crashers regressions with some other changes I'm working on.