[clang.py] Followup to TranslationUnit refactor

* Document index argument in TranslationUnit.from_source
* Add numeric error code to TranslationUnitSaveError string representation
* Use None instead of [] for default argument value in
  TranslationUnit.codeComplete

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156722 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed