[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

include/swift/Serialization/ModuleFormat.h

@@ -55,7 +55,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t VERSION_MINOR = 452; // Last change: nominal types for operators
+const uint16_t VERSION_MINOR = 453; // Last change: borrow always
 
 using DeclIDField = BCFixed<31>;