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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

This seems to have broken LTO linking / introduced some non-determinism. I noticed that the LLVM libc `crt1.o` file was missing a lot of definitions for some reason and traces it back to this. Here's the IR that comes out of `clang` https://godbolt.org/z/4fz35PaPb.

If we take this and then run it through the LTO pass things go wrong.
```console
> ld.lld --lto-emit-llvm start.o -r --lto-partitions=8 -plugin-opt=O3 --lto-CGO3 -o test.o
```

This either results in some definitions being missing or a corrupted LLVM-IR file so it's non-deterministic.
```console
> opt -S test.o
opt: test.o: error: can't skip to bit 50048 from 42112
```

Can I revert this?

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