Update LLVM version 21 for 16.1.X

[smuzaffar]

backport LLVM 21 from CLANG_X IBs to default 16.1.X.
Note that we already have applied clang-tidy changes using LLVM 21 to cmssw master branch. After integrating this version, we will open new PRs to run clang-format using llvm21. We can not open format changes using llvm21 as old llcm 18 will revert those changes. So we have to integrate this first

backport of #10142

[smuzaffar]

please test using full cmssw

[cmsbuild]

A new Pull Request was created by @smuzaffar for branch IB/CMSSW_16_1_X/master.

@akritkbehera, @cmsbuild, @iarspider, @raoatifshad, @smuzaffar can you please review it and eventually sign? Thanks.
@ftenchini, @mandrenguyen, @sextonkennedy you are the release manager for this.
cms-bot commands are listed here

• Backported from [CLANG_X][llvm]Update to version 21.1.4 #10142

[cmsbuild]

cms-bot internal usage

[smuzaffar]

backport of #10142

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-d8e530/51128/summary.html
COMMIT: 9099143
CMSSW: CMSSW_16_1_X_2026-02-04-2300/el8_amd64_gcc13
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmsdist/10322/51128/install.sh to create a dev area with all the needed externals and cmssw changes.

The following merge commits were also included on top of IB + this PR after doing git cms-merge-topic:

• @waredjeb Fix seed column in LayerCluster SoA  cmssw#50017
• @smuzaffar Drop Alignment/Cocoa packages cmssw#50016
• @CMSTrackerDPG HLT digi morphing fix cmssw#49991
• @smuzaffar GCC15: Fix build error array subscript -1 is below array bounds cmssw#50009
• @goblirsc Update tracker alignment algs to new Millepede+GBL versions cmssw#49963
• @AdrianoDee ParkingHH0 2025 Data Workflows cmssw#50028
• @raoatifshad [GCC15]: (alca-db) Fix build warnings in CalibTracker/SiStripQuality … cmssw#50030
• @makortel Escape special characters in most JobReport XML elements cmssw#50024
• @goblirsc Add new "Mille" package to externals, update Millepede-II and GBL  #10310
• @cms-sw SCRAM: proint an error message if there is invalid file/directory in src tree #10321

You can see more details here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-d8e530/51128/git-recent-commits.json
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-d8e530/51128/git-merge-result

Comparison Summary

Summary:

• You potentially removed 165 lines from the logs
• ROOTFileChecks: Some differences in event products or their sizes found
• Reco comparison results: 8 differences found in the comparisons
• DQMHistoTests: Total files compared: 52
• DQMHistoTests: Total histograms compared: 4029600
• DQMHistoTests: Total failures: 8
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 4029572
• DQMHistoTests: Total skipped: 20
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 51 files compared)
• Checked 222 log files, 193 edm output root files, 52 DQM output files
• TriggerResults: no differences found

[smuzaffar]

@makortel @cms-sw/orp-l2 , any objections on getting LLVM21 in 16.1.X normal IBs? Note that once we include it than

• any update/change to opened PRs will run code checks with new LLVM which might ask to do extra formating
• We need to open PRs to formats all files not touched by already open PRs ( see the nearly complete list here [Do not merge] LLVM21 clang-format suggestions cmssw#49293 )

[mandrenguyen]

+1
Good for me!

[makortel]

Fine with me (given that we don't enforce clang-format on production release cycles).

[smuzaffar]

+externals

merging this for tonight's IB

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IB/CMSSW_16_1_X/master IBs (tests are also fine). This pull request will be automatically merged.