[libclang] When parsing with CXTranslationUnit_ForSerialization, make sure to install the ASTWriter that we create as an ASTMutationListener.

Fixes rdar://13833268

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181575 91177308-0d34-0410-b5e6-96231b3b80d8