[llvm] Use MapVector to fix lld thinLTO bep issue. (PR #117551)

[James Henderson via llvm-commits]

jh7370 wrote:

Thanks for the update. Is there a GitHub issue associated with the problem you're trying to fix? If so, please include "Fixes #####" in your PR description.

Just so you know, I'm not familiar with the area, so somebody who is will need to perform the review. Take a look through past contributors to this file and try pinging them on here/adding them as reviewers.

https://github.com/llvm/llvm-project/pull/117551