[llvm] Two DWARF variant part improvements (PR #138953)

[Tom Tromey via llvm-commits]

tromey wrote:

While looking into this code some more today, I found out that more changes will be needed for Ada.  So while this patch is necessary, it's not sufficient.  Not sure it really matters but I thought I'd point it out.  I'll mention it on the discourse thread.

Also I do not have write access, so could someone please land this for me?  Thank you.

https://github.com/llvm/llvm-project/pull/138953