[clang] [Inliner] Improve attribute propagation to callsites when inlining. (PR #66036)

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goldsteinn wrote:

> I don't think 10 commit PRs are helpful. This should be 3 PRs at least. One is trivial, one is probably easy, and then one with the new stuff.

So PR based on PR? The thing is there are dependencies between commits.
My expectation is that I will push commits as they are approved, not the whole PR
at once.

https://github.com/llvm/llvm-project/pull/66036