[clang] [clang] Add noipa attribute (PR #207502)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Adds a `__attribute__((noipa))` attribute to clang, more info on this attribute can be found in #203304.
> 
> We also set the noinline attribute on the targeted function as pointed out in #203304.
> 
> fixes #40819
> 
> Assisted by GPT 5.4 for tests

There was previous discussion on this in https://discourse.llvm.org/t/revisiting-refining-the-definition-of-optnone-with-interprocedural-transformations/58095/11 and I think there was maybe enough push back to warrant a new RFC to ensure everyone is on the same page regarding the design, particularly regarding whether this implies noinline or not. However, CCing folks involved there for opinions in case we think we can avoid an RFC: @jdoerfert @dwblaikie 

https://github.com/llvm/llvm-project/pull/207502