[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.
> 
> Thanks, I have a problem, it seems that I can't add reviewers.

>From the automated greeting post above:

> If you wish to, you can add reviewers by using the "Reviewers" section on this page.
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> If this is not working for you, it is probably because you do not have write permissions for the repository. In which case you can instead tag reviewers by name in a comment by using @ followed by their GitHub username.

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