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

[Danila Malyutin via llvm-commits]

danilaml wrote:

Above breaks treat-warnings-as-errors builds. Not sure if it's due to incorrect attribute usage or due to this (or similar) gcc "bug": https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587

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