Friend functions of private Base class in tuple not accessible when building with XLC.

[rchen20]

• Place base back into private members of tuple.

While the test-kernel-single-loop-ForICount-8-OpenMPTarget test in RAJA (LLNL/RAJA#1207), there was a compilation error in CAMP tuple. I'm not sure if this is the right solution, but making the base tuple_helper class publicly accessible solved the build error (although the test still failed with XLC).

Note: This solution does not solve the clang internal error LLNL/RAJA#1216.

This PR was a band-aid for the problem (#98), but we want to avoid exposing the Base class.

Error:
In file included from /usr/WS1/chen59/allraja/rajaatomicexhaustive/raja_git_atomicexhaustive/build_lc_blueos-xl_omptarget-2021.12.22/test/functional/kernel/single-loop-tile-icount-tcount/test-kernel-single-loop-ForICount-8-OpenMPTarget.cpp:11:
In file included from /usr/WS1/chen59/allraja/rajaatomicexhaustive/raja_git_atomicexhaustive/test/include/RAJA_test-base.hpp:15:
In file included from /usr/WS1/chen59/allraja/rajaatomicexhaustive/raja_git_atomicexhaustive/include/RAJA/RAJA.hpp:33:
In file included from /usr/WS1/chen59/allraja/rajaatomicexhaustive/raja_git_atomicexhaustive/include/RAJA/util/camp_aliases.hpp:31:
/usr/WS1/chen59/allraja/rajaatomicexhaustive/raja_git_atomicexhaustive/tpl/camp/include/camp/tuple.hpp:154:60: error: 'base' is a private member of 'camp::tuple<RAJA::TypedRangeSegment<unsigned long long, long long> >'
return static_cast<tpl_get_store<Tuple, index> const&>(t.base).get_inner();
^

CI Error:
https://lc.llnl.gov/gitlab/radiuss/RAJA/-/jobs/561894