[llvm] Add __attribute__((retain)) to LLVM_DUMP_METHOD (PR #133025)

[Matthias Braun via llvm-commits]

MatzeB wrote:

Though even if code did forget to test for `LLVM_ENABLE_DUMP` you would still not get the retain attribute, so should not be worse than before my patch.

https://github.com/llvm/llvm-project/pull/133025