[llvm] [Attr] Add `noipa` function attribute (PR #203304)

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

Ah yes, I can confirm locally that function specialisation does still run even with `noipa`. (I did notice function specialisation during my manual review of IPO passes, but initially thought it might have been controlled by the logic already changed in IPSCCP. I see now though that specialisation is always called and has its own legality checks.)

Should I also address the larger problem you mention with linkages in this PR, or only add a focused check for the `noipa` attribute (since you said `hasExactDefinition` is too strong) and defer the rest to a separate issue / PR? (I am happy to do either...!)

https://github.com/llvm/llvm-project/pull/203304