Properly handle DMD-internal "reference variables".

Previously, we just had a hack to make ref foreach statements work.
This commit enables them to work in other cases as well, like the
implicit __result variable for functions with out-contracts (which
is such a magic ref variable for ref-returning functions).

Fixes DMD testcase 'testcontracts'.

ir/irvar.cpp

@@ -31,7 +31,6 @@ IrGlobal::IrGlobal(VarDeclaration* v): IrVar(v)
 IrLocal::IrLocal(VarDeclaration* v) : IrVar(v)
 {
     nestedIndex = -1;
-    byref = false;
 }
 
 //////////////////////////////////////////////////////////////////////////////

ir/irvar.h

@@ -28,8 +28,8 @@ struct IrLocal : IrVar
 {
     IrLocal(VarDeclaration* v);
 
-    bool byref;         // Not used for -nested-ctx=array
-    int nestedDepth;    // ditto
+    // Used for hybrid nested context creation.
+    int nestedDepth;
     int nestedIndex;
 };