Put the mechanism in place to track modifications in an AST entity that were committe...

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