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

Nobody has claimed this yet.

- [ ] Place `base` back into private members of `tuple`.

While the test-kernel-single-loop-ForICount-8-OpenMPTarget test in RAJA (https://github.com/LLNL/RAJA/pull/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 https://github.com/LLNL/RAJA/issues/1216.

This PR was a band-aid for the problem (https://github.com/LLNL/camp/pull/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 >'
return static_cast const&>(t.base).get_inner();
^

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

Start in tpl/camp/include/camp/tuple.hpp around the access to t.base at line 154, then inspect the test-kernel-single-loop-ForICount-8-OpenMPTarget test and its XLC CI error. Determine how to keep Base private while allowing the required tuple access, and verify the test builds without exposing Base; the noted clang internal error is separate.

Written by the indexing model from the issue text.