[libclang] Improve AST serialization done by ASTUnit::Save().

The ASTUnit needs to initialize an ASTWriter at the beginning of
parsing to fully handle serialization of a translation unit that
imports modules. Do this by introducing an option to enable it, which
corresponds to CXTranslationUnit_ForSerialization on the C API side.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165717 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed