[clang] [HIP] Remove default `-flto-partitions=8` in the HIP toolchain (PR #203948)

[Pierre van Houtryve via cfe-commits]

Pierre-vh wrote:

> Hard disagree, doing this by default is bad even if it works perfectly fine.

I mean, I am in favor of this landing. I can see why having it on by default is counterproductive. All I was saying is that I was just not in favor of this landing w/ issues known and ignored. If the issues are filed, even if they're not fixed anytime, LGTM.

https://github.com/llvm/llvm-project/pull/203948