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

[Danila Malyutin via llvm-commits]

danilaml wrote:

@MatzeB comment says
```cpp
/// Note that you should also surround dump() functions with
/// `#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)` so they do always
/// get stripped in release builds
```
So I assume not all dump methods are behind ifdefs. Also, wouldn't surprise me if some dump-like functions are annotated with `LLVM_ENABLE_DUMP` so that they would be available in assertion-enabled builds for gdb but they are still used in release builds (so the macro is used so that there would be a definition debugger could use, rather than complaining that it was inlined).

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