[NFC] OwnershipUtils: Add a UsePoint type.

The type is a union of an Operand (a real use) and a SILInstruction (an
implicit use).  Such a type is needed to reflect the fact that with
incomplete lifetimes, values can be implicitly destroyed at the
terminators of blocks in dead end regions (along the vaule's
availability boundary).

lib/SIL/Utils/OwnershipUtils.cpp

@@ -992,6 +992,9 @@ bool BorrowedValue::areWithinExtendedScope(Instructions insts,
   return liveness.areWithinBoundary(insts, deadEndBlocks);
 }
 
+template bool BorrowedValue::areWithinExtendedScope<UsePointInstructionRange>(
+    UsePointInstructionRange insts, DeadEndBlocks *deadEndBlocks) const;
+
 template bool
 BorrowedValue::areWithinExtendedScope<SILInstruction::OperandUserRange>(
     SILInstruction::OperandUserRange insts, DeadEndBlocks *deadEndBlocks) const;