[semantic-sil] Add a new API ManagedValue::transform() for transforming/forwarding a cleanup for a ManagedValue.

In SILGen, we often times want to transform a ManagedValue, while maintaining
the underlying cleanup. The ManagedValue::transform API attempts to formalize
this pattern through the usage of std::move to create a movable rvalue, but
returning a different ManagedValue as a full value. This ensure that the
original ManagedValue is destroyed at end of statement, will allowing the
underlying cleanup to be forwarded.

From an ownership perspective, I think the majority of these cases should
actually recreate the underlying cleanup since in most cases they involve
forwarding. For today though I am using this to just formalize this pattern at
the relevant call sites.

rdar://29791263

Author: gottesmm

lib/SILGen/ManagedValue.h

@@ -123,7 +123,22 @@ class ManagedValue {
   SILValue getValue() const { return valueAndFlag.getPointer(); }
 
   SILType getType() const { return getValue()->getType(); }
-  
+
+  /// Transform the given ManagedValue, replacing the underlying value, but
+  /// keeping the same cleanup.
+  ///
+  /// For owned values, this is equivalent to forwarding the cleanup and
+  /// creating a new cleanup of the same type on the new value. This is useful
+  /// for forwarding sequences.
+  ///
+  /// For all other values, it is a move.
+  ManagedValue transform(SILValue newValue) && {
+    assert(getValue().getOwnershipKind() == newValue.getOwnershipKind() &&
+           "New value and old value must have the same ownership kind");
+    ManagedValue M(newValue, isLValue(), getCleanup());
+    *this = ManagedValue();
+    return M;
+  }
 
   CanType getSwiftType() const {
     return isLValue()