[llvm] Add __attribute__((__retain__)) to LLVM_ATTRIBUTE_USED (PR #133025)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Some of our internal builds use `-ffunction-sections -Wl,--gc-sections` and I just noticed that we need an additional `__attribute__((retain))` to keep the LLVM dump functions around.
> 
> Is there any downside to using this attribute in LLVM?

Perhaps we should just add retained to `LLVM_DUMP_METHOD`?

This will enable USED functions to be garbage-collectable on ELF platforms, where this behavior has been consistently present.

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