GPU: Better workaround for clang problem with static constexpr members of references

[davidrohr]

Previous workaround just removed the constexpr optimization for clang.
This new version uses some template magic, which works with clang and c++20.
Unfortunately, does not work for OpenCL, which is at c++17, nothing we can do.
With c++23, we will be able to revert this, and just access the reference directly.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 7fa0a86 at 2025-05-14 17:39:

## sw/BUILD/O2Physics-latest/log
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:


## sw/BUILD/O2-RTC-test-latest/log
/tmp/o2cagpu_rtc_131_382498961_6.hip:17298:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_6.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_6.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_6.hip:17440:39: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_6.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_54.hip:17298:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_54.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_54.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_54.hip:17440:39: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_54.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_38.hip:17298:33: error: constexpr if condition is not a constant expression
 17298 | /tmp/o2cagpu_rtc_131_382498961_11.hip:17298:33: error: constexpr if condition is not a constant expression
 17163 |   const/tmp/o2cagpu_rtc_131_382498961_11.hip:17303:37: error: constexpr if condition is not a constant expression
17303 |             if GPUC/tmp/o2cagpu_rtc_131_382498961_38.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_11.hip:17416:33: error: constexpr if condition is not a constant expression
 17416 |         if GPUCA_RTC_CONSTEXPR ((param.par).dodEdx)/tmp/o2cagpu_rtc_131_382498961_38.hip:17416:33: error: constexpr if condition is not a constant expression
17416 |         if GPUCA_RTC_CONSTEXPR ((param.par).dodEd/tmp/o2cagpu_rtc_131_382498961_11.hip:17440:39: error: constexpr if condition is not a constant expression
17163 |   const GPUParam& __restrict__ param = merger-/tmp/o2cagpu_rtc_131_382498961_38.hip:17440:39: error: constexpr if condition is not a constant expression
17440 |               if GPUCA_RTC_CONSTEXPR ((param.rec.tpc)./tmp/o2cagpu_rtc_131_382498961_11.hip:17474:29: error: constexpr if condition is not a constant expression
:17303:37: error: constexpr if condition is not a constant expression
 17303 |             if GPUCA_RTC_CONSTEXPR ((p/tmp/o2cagpu_rtc_131_382498961_38.hip:17474:29: error: constexpr if condition is not a constant expression
aram.rec.tpc).dEdxClusterRejectionFlagMask != (param.rec.tpc).dEdxClusterRejection/tmp/o2cagpu_rtc_131_382498961_10.hip:17298:33: error: constexpr if condition is not a constant expression
17298 |         if GPUCA_RTC_CONSTEXPR ((/tmp/o2cagpu_rtc_131_382498961_61.hip:17298:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_10.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_61.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_88.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_88.hip:17440:39: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_10.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_61.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_88.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_10.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_61.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_89.hip:17298:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_89.hip:17303:37: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_89.hip:17416:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_89.hip:17440:39: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_89.hip:17474:29: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_83.hip:17298:33: error: constexpr if condition is not a constant expression
/tmp/o2cagpu_rtc_131_382498961_83.hip:17303:37: error: constexpr if condition is not a constant expression
[0 more errors; see full log]

Full log here.