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/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index c7b014e..313c2ea 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -20,6 +20,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExternalASTSource.h"
+#include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/SourceManager.h"
@@ -5200,6 +5201,8 @@
 
 void ExternalASTSource::PrintStats() { }
 
+ASTMutationListener::~ASTMutationListener() { }
+
 
 //===----------------------------------------------------------------------===//
 //                          Builtin Type Computation