Fixed crash in FindUsages on a dirty file in a large project, which is saved
while the FindUsages is still running.

Now, the ASTVisitor constructor takes a valid reference to a TranslationUnit.

Done with Erik Verbruggen

Ensure the code model receives file names with clean paths ('/')
and fix paths in the relevant places of the code model. Pass on clean
paths from Cpp reference find and display them correctly in the search
window tooltip.

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>