Significantly simplify CGExprAgg's logic about ignored results:
if we want to ignore a result, the Dest will be null.  Otherwise,
we must copy into it.  This means we need to ensure a slot when
loading from a volatile l-value.

With all that in place, fix a bug with chained assignments into
__block variables of aggregate type where we were losing insight into
the actual source of the value during the second assignment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159630 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGValue.h b/lib/CodeGen/CGValue.h
index dd9208f..a46f313 100644
--- a/lib/CodeGen/CGValue.h
+++ b/lib/CodeGen/CGValue.h
@@ -389,7 +389,8 @@
     return AV;
   }
 
-  static AggValueSlot forLValue(LValue LV, IsDestructed_t isDestructed,
+  static AggValueSlot forLValue(const LValue &LV,
+                                IsDestructed_t isDestructed,
                                 NeedsGCBarriers_t needsGC,
                                 IsAliased_t isAliased,
                                 IsZeroed_t isZeroed = IsNotZeroed) {