[sil] Add a new CastConsumptionKind called BorrowAlways.

This means that:

1. SILGenPattern always borrows the object before it emits a case.
2. Any cast with this cast has a +0 result.

NOTE: That one can not use this with address types (so we assert if you
pass this checked_cast_addr_br).
NOTE: Once we have opaque values, checked_cast_br of a guaranteed value will
lower to a copy + checked_cast_addr_br (assuming the operation is a consuming
cast). To make sure this does not become a problem in terms of performance, we
will need a pass that can transform SILGenPattern +0 cases to +1 cases. This is
something that we have talked about in the past and I think it is reasonable to
implement.

This is an incremental commit towards fixing SILGenPattern for ownership.

rdar://29791263

lib/Serialization/SerializeSIL.cpp

@@ -85,6 +85,8 @@ static unsigned toStableCastConsumptionKind(CastConsumptionKind kind) {
     return SIL_CAST_CONSUMPTION_TAKE_ON_SUCCESS;
   case CastConsumptionKind::CopyOnSuccess:
     return SIL_CAST_CONSUMPTION_COPY_ON_SUCCESS;
+  case CastConsumptionKind::BorrowAlways:
+    return SIL_CAST_CONSUMPTION_BORROW_ALWAYS;
   }
   llvm_unreachable("bad cast consumption kind");
 }