[clang] [clang][LifetimeSafety] Short-circuit joins of identical immutable containers (PR #208908)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 approved this pull request.

Great catch. This is amazing.

This obsoletes `generate_cpp_switch_fan_out_test` but I think  we can still let it stay to ensure this doesn't blow up again.

https://github.com/llvm/llvm-project/pull/208908