Clang: Support second translation unit

A TranslationUnit is owned by TranslationUnits now. TranslationUnits
allows to add another TranslationUnit and to update/query the recently
and previously parsed translation unit.

This does not change any behavior yet.

Change-Id: I8a2f0cc05d3e51bf739dd5d7c4da14b54147f3ab
Reviewed-by: David Schulz <david.schulz@qt.io>

tests/unit/unittest/skippedsourceranges-test.cpp

@@ -99,6 +99,7 @@ struct Data {
                       {},
                       documents};
     TranslationUnit translationUnit{filePath,
+                                    filePath,
                                     document.translationUnit().cxIndex(),
                                     document.translationUnit().cxTranslationUnit()};
 };