[clang] [llvm] [PowerPC] Fix codegen for transparent_union function params (PR #101738)

[Amy Kwan via cfe-commits]

https://github.com/amy-kwan commented:

I'm not an expert here, but I think the change you made makes sense since the `Ty` is supposed to be the first field.

Also, might be a dumb question, I saw we have 32-bit Linux run lines, but is that still worth testing?

https://github.com/llvm/llvm-project/pull/101738