[lld] [llvm] [DTLTO] Make temporary file handling consistent (PR #176807)

[Ben Dunbobbin via llvm-commits]

bd1976bris wrote:

> > TIL about "#" and FileCheck's numeric substitution blocks, this is a nice shorthand and I'm not sure why I didn't know about it! However, a lot of the changes to use it throughout the tests seem unrelated to the source code changes. Can you refactor those changes out into a separate NFC patch and go ahead and commit that separately? It would make the test changes easier to review.
> 
> Thanks for looking at this - I can't take credit @MaskRay pointed out that the FileCheck matches could be tightened up in a recent LLD side change. I'll factor this stuff out into it's own PR.

Done in #177282. I have rebased this patch on top now and addressed your other comments. Please take another look.

https://github.com/llvm/llvm-project/pull/176807