[llvm] Fix `llvm/test/DebugInfo/Generic/discriminated-union.ll` on big-endian targets (PR #125849)

[Rainer Orth via llvm-commits]

rorth wrote:

I just tried it successfully on `sparcv9-sun-solaris2.11`.  Thanks.

`target-byteorder-*-endian` is nice, btw.  This would be helpful in other parts of the tree (like `flang` in particular).

https://github.com/llvm/llvm-project/pull/125849