restore parseTranslationUnit() and TUManager::Settings

Using clang_createTranslationUnitFromSourceFile() alone seemed to have
degraded completion performance because preamble weren't enabled.

CXTranslationUnit_CacheCompletionResults because otherwise the problem
that a1b045c7f5f0377748fe2deaa31b70cd3e13379d was solving is coming
back. It looks like libclang's completion results caching may be a bit
broken? Needs further investigation.

Restoring the options also means that
CXTranslationUnit_IncludeBriefCommentsInCodeCompletion will be used
again and hopefully that will fix
https://github.com/Sarcasm/irony-mode/issues/116