Put the mechanism in place to track modifications in an AST entity that were committed after
its initial creation/deserialization and store the changes in a chained PCH.

The idea is that the AST entities call methods on the ASTMutationListener to give notifications
of changes; the PCHWriter implements the ASTMutationListener interface and stores the incremental changes
of the updated entity. WIP

llvm-svn: 117235
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index d60a067..483352f 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -210,6 +210,10 @@
   return getTranslationUnitDecl()->getASTContext();
 }
 
+ASTMutationListener *Decl::getASTMutationListener() const {
+  return getASTContext().getASTMutationListener();
+}
+
 bool Decl::isUsed(bool CheckUsedAttr) const { 
   if (Used)
     return true;