This improves moc's support for nested inline namespaces, so that code
containing them will not break compilation.
For simplicity, we allow nested inline namespaces even in C++17 mode
(the actual C++ compiler will reject the code anyway, and probably with
a better error message than moc could output).
moc still has no real awareness how inline namespaces work, but that is
a preexisting issue.
Pick-to: 6.4 6.2
Fixes: QTBUG-106920
Change-Id: I7b415a99133575f101bc81d01d4670a5f752917f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>