iwyu: Update mappings #34460

1. 
   hebasto commented at 11:57 am on January 30, 2026: member

   This PR:

   1. Removes mappings that have been backported upstream.
   2. Adds a new temporary mapping to work around upstream issue.
   3. Document the existing mappings for libc symbols.
2. iwyu: Drop backported mapping

   See https://github.com/include-what-you-use/include-what-you-use/pull/1706.

   37de7d1910
3. iwyu: Add temporary mapping to work around upstream bug 91824646c5
4. iwyu: Document mappings for libc symbols 9c839aa9e3
5. 
   DrahtBot commented at 11:57 am on January 30, 2026: contributor

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6. 
   maflcko commented at 12:18 pm on January 30, 2026: member

   lgtm ACK 9c839aa9e3db30e2fa7d45b087a13bdd86b2a085
7. sedited approved
8. 
   sedited commented at 8:41 am on January 31, 2026: contributor

   ACK 9c839aa9e3db30e2fa7d45b087a13bdd86b2a085
9. sedited merged this on Jan 31, 2026
10. sedited closed this on Jan 31, 2026

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11. hebasto deleted the branch on Jan 31, 2026

Contributors
hebasto DrahtBot maflcko sedited