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/Sema/IdentifierResolver.cpp b/lib/Sema/IdentifierResolver.cpp
index 4c6898c..4d62cab 100644
--- a/lib/Sema/IdentifierResolver.cpp
+++ b/lib/Sema/IdentifierResolver.cpp
@@ -304,7 +304,7 @@
     for (Decl::redecl_iterator RD = New->redecls_begin(), 
                             RDEnd = New->redecls_end();
          RD != RDEnd; ++RD) {
-      if (RD == Existing)
+      if (*RD == Existing)
         return DMK_Replace;
         
       if (RD->isCanonicalDecl())