[ownership] Ban ownership constraints with preferredKind OwnershipKind::Unowned.

It used to be that we had certain instructions that could only have operands
with OwnershipKind::{None,Unowned}. This changed with the addition of
OperandOwnership which caused us to begin accepting owned and guaranteed in all
positions where we accept unowned values with the proviso that certain
forwarding instructions are now allowed to force their result to be unowned even
though their argument is owned or guaranteed.

This setup massively simplifies RAUWing values in OSSA by eliminating the need
to insert conversion unchecked_ownership_conversion instructions from
owned->unowned when replacing an unowned value with an owned or guaranteed
value. Now, these uses can just take the owned or guaranteed value directly and
we wish to maintain that property. Thus we are banning it in SIL by writing a
verifier check.

Author: gottesmm

lib/SIL/Verifier/SILVerifier.cpp

@@ -1133,6 +1133,18 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
                         UseLifetimeConstraint::NonLifetimeEnding,
                 "In non-ossa all non-type dependent operands must have a "
                 "constraint of Any, NonLifetimeEnding");
+      } else {
+        // Perform some structural checks on the operand if we have ownership.
+
+        // Make sure that our operand constraint isn't Unowned. There do not
+        // exist in SIL today any instructions that require an Unowned
+        // value. This is because we want to always allow for guaranteed and
+        // owned values to be passed as values to operands that take unowned.
+        auto constraint = operand.getOwnershipConstraint();
+        require(constraint.getPreferredKind() != OwnershipKind::Unowned,
+                "Operand constraint should never have an unowned preferred "
+                "kind since guaranteed and owned values can always be passed "
+                "in unowned positions");
       }
     }