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

[Nikita Popov via llvm-commits]

nikic wrote:

Looks like another thing that bypasses noipa is function specialization. However, I think this is actually indication of a larger bug. Consider this case: https://llvm.godbolt.org/z/vqd6q93o3 This gets specialized despite the function having weak linkage. This is incorrect if the weak definition we see is not the prevailing definition. Function specialization should check that the function is !isInterposable() (hasExactDefinition() shouldn't be necessary though).

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