[clang] [Sema] Implement fix as suggested by FIXME (PR #143142)

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https://github.com/Sirraide commented:

I’m not sure this is better? Having to manually `delete` these is... not great. Have you benchmarked whether this improves memory usage or makes Clang faster?

See also this comment by Aaron on the issue you linked: https://github.com/llvm/llvm-project/issues/143129#issuecomment-2949619132

https://github.com/llvm/llvm-project/pull/143142