Have LoopConvert use 'auto &&' where necessary

For iterators where the dereference operator returns by value, LoopConvert
should use 'auto &&' in the range-based for loop expression.

If the dereference operator returns an rvalue reference, this is deemed too
strange and the for loop is not converted.

Moved test case from iterator_failing.cpp to iterator.cpp and added extra
tests.

Fixes PR15437.

Reviewer: gribozavr
llvm-svn: 176631