[silgen] Always ensure that we have a +1 value when emitting reabstraction thunks.

The machinery assumes that it will always have a +1 value. I am attempting to do
the minimal fix here for cherry-picking purposes.

There isn't a test case with this commit since the immutable address verifier
(in the next commit) verifies that this is done correctly. The specific test
that will trip is vtable_thunks_reabstraction.swift.

rdar://50212579

Author: gottesmm

lib/SILGen/SILGenPoly.cpp

@@ -1116,9 +1116,18 @@ namespace {
 
           // Load if necessary.
           if (elt.getType().isAddress()) {
+            // This code assumes that we have each element at +1. So, if we do
+            // not have a cleanup, we emit a load [copy]. This can occur if we
+            // are translating in_guaranteed parameters.
+            IsTake_t isTakeVal = ([&] {
+              if (elt.isPlusZero()) {
+                return IsNotTake;
+              }
+              return IsTake;
+            }());
             elt = SGF.emitLoad(Loc, elt.forward(SGF),
-                               SGF.getTypeLowering(elt.getType()),
-                               SGFContext(), IsTake);
+                               SGF.getTypeLowering(elt.getType()), SGFContext(),
+                               isTakeVal);
           }
         }