[clang] [clang][docs] Reflect the implementation status for P2280R4 (PR #127166)

[Vlad Serebrennikov via cfe-commits]

================
@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -std=c++23 -verify=expected,nointerpreter %s
 // RUN: %clang_cc1 -std=c++23 -verify %s -fexperimental-new-constant-interpreter
 
+// FIXME: P2280R4 should be backported. Run this in C++11 and later modes.
----------------
Endilll wrote:

On top of this FIXME, add RUN lines for C++11 mode, then add expected directives for whatever comes out of it. So that if someone accidentally makes it work, it doesn't go unnoticed.

https://github.com/llvm/llvm-project/pull/127166