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

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

> Missing test coverage.

There is a tiny unit test checking the attribute's effect on `GlobalValue::isDefinitionExact`, but I understand that's quite a small amount of coverage. I'll add additional IR level tests of the feature's behaviour (adapted from the [related issue](https://github.com/llvm/llvm-project/issues/40819)).

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