[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. 

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