SIL: Fix DynamicSelfType references from inside init accessor
We need the self metatype parameter to correctly lower
DynamicSelfType in IRGen, so plumb this through to all
calls of init accessors, and inside the prolog of an
init accessor definition.
This does not break the public ABI, because init
accessors are never public. Also for value types, the
metatype is thin, so it should not change generated
code.
For classes we need the metatype in the general case
because of `Self`, but hopefully in most cases the
init accessor can be inlined away and the value_metatype
instruction subject to dead code elimination.
Fixes rdar://problem/119822466.
