To correctly call designated super class initializers the designated
intializer (and not the allocator) is dynamically replaceable.
Convenience allocators are dynamically replaceable as before.
We weren't doing much validation when dynamically replacing storage
declarations, and has an assert() that should be an error. Clean up this
area a bit, dealing with simple ambiguities (i.e., there are two
properties or subscripts with different type signatures; pick the
matching one) and reporting an error when there is a true ambiguity.
Fixes rdar://problem/46737657.
