The runtime hash table for protocol conformances is keyed by (type, protocol),
where the type can be either a type metadata pointer or a type context
descriptor. The latter is preferred for generic and resilient types, because
a single entry in the cache can work for any instantiation.
However, not all type metadata has a corresponding type context descriptor.
For example, a class that is dynamically subclassed by the Objective-C
runtime won't have a type context descriptor. In such cases, our key
into the hash table was (NULL, protocol) leading to mistaken
conformances for different dynamically-subclassed types.
Introduce a NULL check for the type of the hash table key, which
illustrates the problem in a number of existing tests that exercise
the runtime, and teach the runtime to use the type context descriptor
as the key only when it's non-NULL, falling back to the type metadata
pointer otherwise.
Fixes rdar://problem/38053213.
