SILGen: Fix crash when emitting foreign-to-native thunk for allocating init in -swift-version 5
This comes up when we import a static factory method as a convenience init.
The thunk was using DynamicSelfType as the type of a basic block argument,
because that was the type of the 'self' parameter in -swift-version 5.
Fixes <rdar://problem/44242156>.