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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Oof, this is tough (at least for me), we seem to have reasonably strong support for `noipa` implying `noinline` but we also have someone pointing out that they've already needed these to be separated and that the workaround with `always_inline` may not be sufficient in all circumstances.

In terms of compatibility with GCC, they document:

>  This attribute implies noinline, noclone and no_icf attributes

We don't expose `noclone` or `no_icf` as Clang attributes, but does LLVM have the concepts to map to in IR? e.g., is the argument about compatibility with GCC somewhat moot because we're not going to be compatible anyway?

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