[clang] [Clang] allow `` `@$ `` in raw string delimiters in C++26 (PR #93216)

[Younan Zhang via cfe-commits]

================
@@ -111,6 +111,14 @@ def warn_cxx98_compat_raw_string_literal : Warning<
   "raw string literals are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 
+def warn_cxx26_compat_raw_string_literal_character_set : Warning<
+  "'%0'in a raw string literal delimiter is incompatible "
+  "with standards before C++2c">,
+  InGroup<CXXPre26Compat>, DefaultIgnore;
+def ext_cxx26_raw_string_literal_character_set : Extension<
+  "'%0'in a raw string literal delimiter is a C++2c extension">,
----------------
zyn0217 wrote:

```suggestion
  "'%0' in a raw string literal delimiter is a C++2c extension">,
```

https://github.com/llvm/llvm-project/pull/93216