[clang] [PowerPC] Diagnose invalid combination with Altivec, VSX and soft-float (PR #79109)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> cc @MaskRay

The change is reasonable. I'd indent continuation lines by 2 spaces, but probably not worth the churn.

https://github.com/llvm/llvm-project/pull/79109