swift234

Re-apply "Protocol conformances can now reference resilient value types"

This comes with a fix for a null pointer dereference in _typeByName()
that would pop with foreign classes that do not have a
NominalTypeDescriptor.

Also, I decided to back out part of the change for now, where the
NominalTypeDescriptor references an accessor function instead of a
pattern, since this broke LLDB, which reaches into the pattern to
get the generic cache.

Soon we will split off the generic cache from the pattern, and at
that time we can change the NominalTypeDescriptor to point at the
cache. But for now, let's avoid needless churn in LLDB by keeping
that part of the setup unchanged.