LLDB: Fix display of bases with multiple inheritance.

It was only showing the "last" one because the inames were the same
(currentIName.None).
Do not overwrite iname of sub item in 'enterSubItem' if it already was
set.

Task-number: QTCREATORBUG-11109
Change-Id: I74581f62d47ff0c33650769c3ed66e24124d1092
Reviewed-by: hjk <hjk121@nokiamail.com>