Introduce a redecl_iterator in Decl class, so that we can do a "iterate over all declarations of the same decl" without knowing the exact type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76298 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index e59d632..6b1f830 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -565,7 +565,7 @@
 
 void 
 FunctionDecl::setPreviousDeclaration(FunctionDecl *PrevDecl) {
-  Redeclarable<FunctionDecl>::setPreviousDeclaration(PrevDecl);
+  redeclarable_base::setPreviousDeclaration(PrevDecl);
 
   if (FunctionTemplateDecl *FunTmpl = getDescribedFunctionTemplate()) {
     FunctionTemplateDecl *PrevFunTmpl