Cleanup of TranslationUnit and CodeCompleteResults.

[eidheim]

@zalox: hope you can look over and merge this soon. The main problem was a bit confusing constructors and methods, and the std::map that forced us to previously do a copy of the source buffer (which is expensive). This is fixed now.

Mind that only one unsaved buffer is used now when calling the libclang functions. I really do not see any case where sending several unsaved files to libclang is a good idea, at least we would never do that in juCi++. If this is needed anyway (highly doubtful), I suggest that we try find a better solution than std::map, in order to avoid copy.

[zalox]

I used this approach to eliminate some common use-cases.
For instance:

1. Edit header file (add definitions)
2. Jump to implementation file
3. Now I expect accurate completions from the header file, which I won't get unless I remember to save

[eidheim]

Reparsing based on a changed header that is not saved is a bad idea since libclang have to recreate its translation unit cache and this is very slow. Also if a header file is changed and not saved, the errors and warnings would not reflect what would happen if you run compile.