...instead of crashing. Also drop the class if its generic
requirements depend on a type that can't be loaded (instead of
crashing).
rdar://problem/50125674
This is enough to let the test case in rdar://problem/40899824 pass,
and any callers of this function already need to be able to handle a
nullptr result. There's a lot more work to do in this area, but it's
nice to get the simple things working again.