Fix CODEOWNERS error, remove Lanza from ClangIR owners (#207294)
The github project reports:
Unknown owner on line 39: make sure <name> exists and has write access
to the repository
I assume Nathan's commit access lapsed and he has the `triage` role now.
I added a comment saying he is an emeritus owner. This is reversible,
and I assume if he needs or wants write access, we can revisit this in
the future.