Revert Decl's iterators back to pointer value_type rather than reference value_type

In addition, I've made the pointer and reference typedef 'void' rather than T*
just so they can't get misused. I would've omitted them entirely but
std::distance likes them to be there even if it doesn't use them.

This rolls back r155808 and r155869.

Review by Doug Gregor incorporating feedback from Chandler Carruth.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158104 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/LayoutOverrideSource.cpp b/lib/Frontend/LayoutOverrideSource.cpp
index ca1dcd3..eb7865e 100644
--- a/lib/Frontend/LayoutOverrideSource.cpp
+++ b/lib/Frontend/LayoutOverrideSource.cpp
@@ -174,7 +174,7 @@
     if (NumFields >= Known->second.FieldOffsets.size())
       continue;
     
-    FieldOffsets[&*F] = Known->second.FieldOffsets[NumFields];
+    FieldOffsets[*F] = Known->second.FieldOffsets[NumFields];
   }
   
   // Wrong number of fields.