SIL: explicitly instantiate the Operand move ctor

GCC 7 does not like the missing move constructor and does not implicitly
synthesize one.  Explicitly indicate that this should be synthesized.

Author: compnerd

include/swift/SIL/SILValue.h

@@ -586,6 +586,8 @@ class Operand {
   Operand(const Operand &use) = delete;
   Operand &operator=(const Operand &use) = delete;
 
+  Operand(Operand &&) = default;
+
   /// Return the current value being used by this operand.
   SILValue get() const { return TheValue; }