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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Thinking on this, might just be a very poor interaction between `-lto-partitions` and `-lto-emit-llvm`. Since it splits it up.

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