[llvm] [rtsan] Handle attributed IR function declarations (PR #169577)

[via llvm-commits]

davidtrevelyan wrote:

> Approved as written! I lean slightly to breaking the NFC into another commit but not strongly enough to block based on it
> 
> Thanks for investigating the other sanitizers. Looks like there is good precedence for this type of check

@cjappl agreed on breaking the NFC into another commit. I've amended this branch to only do enough to fix the bug, adding just the check for the empty function, and we'll tackle the preserved analyses in a separate PR. I think it warrants a little more thought. Thanks for the suggestion!

https://github.com/llvm/llvm-project/pull/169577