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

[Matthias Braun via llvm-commits]

MatzeB wrote:

@aleks-tmb @danilaml @MaskRay Go with #133793 then?

Just dropping the attribute for GCC builds for now... (I suspect there is more folks building with `-Werror` or folks wanting `-Wattributes` enabled than folks who actually need the `retain` attribute given that we haven't had it in LLVM until last week).

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