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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117235 91177308-0d34-0410-b5e6-96231b3b80d8