swift234

ArchetypeBuilder: Allow generic signature superclass constraints to contain type parameters

There was a diagnostic to catch these, but it wasn't triggered
reliably, and it sounds like users were already relying on this
feature working in the few cases where it did.

So instead, just map an archetype's superclass into context
when building the archetype.

Recursion is still not allowed and is diagnosed, for example
<T, U where T : C<U>, U : C<T>>.

Note that compiler_crashers_fixed/00022-no-stacktrace.swift no
longer produces a diagnostic in Sema, despite the fact that the
code is invalid. It does diagnose in IRGen when we map the
type into context. Diagnosing in Sema requires fixing the
declaration checker to correctly handle recursion through a
generic signature. Right now, if recursion is detected, we bail
out, but do not always diagnose. Alternatively, we could
prohibit unbound generic types from appearing in generic
signatures.

This is a more principled fix for rdar://problem/24590570.