Resolve build issues for NVHPC

[mingxwa]

This PR resolves #180

Changes

• NVHPC won't generate the optimal memory layout for proxy when no dispatch is specified, causing static assertions to fail. Although this is a QoI issue for the compiler, we have worked around this by specifying a different base type for proxy in this case.
• NVHPC treats inline const variable as constexpr, causing static_assert(!pro::facade<BadFacade_BadConstraints_NotConstant>) to fail. This should be a bug in NVHPC. I simply worked around the issue in this change by splitting the definition of the variable from its declaration.
• NVHPC creates an additional copy for the value when using static_cast, causing ProxyLifetimeTests.Test_DirectConvension_Lvalue and ProxyLifetimeTests.Test_DirectConvension_Rvalue to fail. This is a bug in the existing unit test implementation since copy elision is not guaranteed for static_cast.
• Modified tests/CMakeLists.txt to exclude freestanding test cases from unsupported compilers like NVHPC.
• Suppressed some "unused variable" warnings from NVHPC that are intended in the unit tests.

Notes

• Manually tested with NVHPC 24.9. Ultimately, we should create a separate pipeline for NVHPC in the future.