[clang] [clang][AMDGPU] Enable module splitting by default (PR #128509)

[Pierre van Houtryve via cfe-commits]

Pierre-vh wrote:

I'll get it tested internally ASAP. I wasn't confident in the driver changes so I was waiting for more feedback before doing it
I'll update once I get results

https://github.com/llvm/llvm-project/pull/128509