python bindings: Use python Diagnostics as function arguments

This improves the readability of the code and fixes one testsuite bug.

The bug happend, because we only stored the pointer to the diagnostic in the
FixIt iterator, but not the python Diagnostic object. So it could happen that
the FixIt iterator still exists, but the python Diagnostic object is freed.
However, as the python Diagnostic is freed the pointer to the diagnostic is also
freed and the FixIt iterator is referencing a freed pointer.

llvm-svn: 124952