Don't follow __assume(false) with std::terminate in NDEBUG builds

Having `std::terminate` as the backstop after `__assume(false)`
would trigger W4702 (unreachable code) with MSVC. As we want to
keep `__assume(false)` for the optimization hint in NDEBUG builds,
we have to avoid mixing in `std::terminate` for those builds.

Fixes #3007

Author: horenmar

src/catch2/internal/catch_unreachable.hpp

@@ -39,9 +39,13 @@ namespace Catch {
             __assume( false );
 #        elif defined( __GNUC__ )
             __builtin_unreachable();
+#        else // vv platform without known optimization hint
+            std::terminate();
 #        endif
-#    endif // ^^ NDEBUG
+#    else  // ^^ NDEBUG
+            // For non-release builds, we prefer termination on bug over UB
             std::terminate();
+#    endif //
         }
 
     } // namespace Detail