[Runtime] Properly form generic arguments for metadata-to-demangle-tree.

[DougGregor]

When mapping from type metadata to a demangle tree, fill in the complete
set of generic arguments. Most of the effort here is in dealing with
extensions that involve same-type constraints on a generic parameter, e.g.,

extension Array where String == Element { }
extension Dictionary where Key == Value { }

In such cases, the metadata won’t contain generic arguments for every
generic parameter. Rather, the generic arguments for non-key generic
parameters will need to be computed based on the same-type requirements
of the context. Do so, and eliminate the old hacks that put the generic
arguments on the innermost type. We don’t need them any more.

Part of rdar://problem/37170296.

[DougGregor]

@swift-ci please smoke test