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

[Aaron Ballman via cfe-commits]

================
@@ -1043,6 +1043,16 @@ the opposite of inlining. It can help to reduce code size.
   }];
 }
 
+def NoIPADocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+This function attribute suppresses interprocedural optimization involving the
+annotated function. Clang lowers it to LLVM IR's ``noipa`` function attribute
+and sets the ```noinline`` function attribute as well, unless always_inline
+is specified.
----------------
AaronBallman wrote:

Let's not mention LLVM IR attributes because Clang users won't be familiar with those. Can you reword it in terms of what the LLVM IR semantics are in practice?
```suggestion
annotated function. Clang lowers it to LLVM IR's ``noipa`` function attribute
and sets the ```noinline`` function attribute as well, unless ``always_inline``
is specified.
```

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