Rename new tuning API policies and fix MSVC warning

[bernhardmgruber]

This followed a small internal discussion on naming conventions.

arch policies -> policy selector
Drop _policy in members
reduce_arch_policy -> reduce_policy
Drop and adapt some comments

As a drive-by:

• adds /diagnostics:caret for MSVC to improve diagnostics
• a different attempt to an MSVC warning which libcu++: silence msvc+nvcc12.9 warning plaguing c.parallel. #7144 tried to fix, but the issue resurfaces in this PR

[bernhardmgruber]

This CI error while compiling cccl.c.parallel.test.histogram.dir/test_histogram.cpp.obj seems a bit odd:

C:\cccl\libcudacxx\include\cuda/std/__complex/roots.h(84): error C2220: the following warning is treated as an error
  C:\cccl\libcudacxx\include\cuda/std/__complex/roots.h(84): warning C4554: '<<': check operator precedence for possible error; use parentheses to clarify precedence

[miscco]

This CI error while compiling cccl.c.parallel.test.histogram.dir/test_histogram.cpp.obj seems a bit odd:

C:\cccl\libcudacxx\include\cuda/std/__complex/roots.h(84): error C2220: the following warning is treated as an error
  C:\cccl\libcudacxx\include\cuda/std/__complex/roots.h(84): warning C4554: '<<': check operator precedence for possible error; use parentheses to clarify precedence

yeah @griwes stumbled on this, looks like something with nvcc eating some of the parenthesis when generating host code

[github-actions]

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