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/AST/ItaniumMangle.cpp b/lib/AST/ItaniumMangle.cpp
index 0c9028e..e974ade 100644
--- a/lib/AST/ItaniumMangle.cpp
+++ b/lib/AST/ItaniumMangle.cpp
@@ -1032,17 +1032,14 @@
   
   for (RecordDecl::field_iterator I = RD->field_begin(), E = RD->field_end();
        I != E; ++I) {
-    const FieldDecl *FD = &*I;
+    if (I->getIdentifier())
+      return *I;
     
-    if (FD->getIdentifier())
-      return FD;
-    
-    if (const RecordType *RT = FD->getType()->getAs<RecordType>()) {
+    if (const RecordType *RT = I->getType()->getAs<RecordType>())
       if (const FieldDecl *NamedDataMember = 
           FindFirstNamedDataMember(RT->getDecl()))
         return NamedDataMember;
     }
-  }
 
   // We didn't find a named data member.
   return 0;