[core] Make some constants constexpr

[vepadulano]

Now that the machinery of #14287 is available, we can begin the process of making constants in ROOT constexpr to get the benefit of a faster initialization. I began with TString::kNPOS and the ones in RTypesCore.h, but do let me know if you see other candidates for the same procedure.

[phsft-bot]

Starting build on ROOT-performance-centos8-multicore/soversion, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac12arm/cxx20, windows10/default
How to customize builds

[github-actions]

Test Results

     9 files       9 suites   1d 20h 26m 35s ⏱️
 2 490 tests  2 490 ✅ 0 💤 0 ❌
21 378 runs  21 378 ✅ 0 💤 0 ❌

Results for commit 5a4c096.

♻️ This comment has been updated with latest results.

[vepadulano]

@phsft-bot build

[phsft-bot]

Starting build on ROOT-performance-centos8-multicore/soversion, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac12arm/cxx20, windows10/default
How to customize builds

[phsft-bot]

Starting build on ROOT-performance-centos8-multicore/soversion, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac12arm/cxx20, windows10/default
How to customize builds

[vepadulano]

For our reference, these are the usual sanity checks

$: strace -z -f -o openat.log -e trace=open,openat python3 -c 'import ROOT;print(ROOT.gErrorIgnoreLevel);print(ROOT.kError)'
-1
3000
$: grep openat openat.log | wc -l
406
$: nm -C $ROOTSYS/lib/*.so | grep TString::kNPOS | wc -l
0