Clean up Class::GetDescriptor().

Pull in helper functions Class::GetArrayDescriptor() and
ClassLinker::GetDescriptorForProxy(), avoid deep recursion
for multidimensional arrays and avoid a read barrier for
GetName() for proxy classes.

Also avoid recursion in Class::DescriptorEquals().

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I04201294434a00eb2f91c66403af45f715941be4