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

[via cfe-commits]

amilendra wrote:

Hello @cor3ntin 
This patch has resulted in strings that were previously rejected are now accepted by the compiler.
https://godbolt.org/z/c1jcGv76z

```
int main() {
  (void) R"\t()\t";
  (void) R"\f()\f";
  (void) R"\n()\n";
  (void) R"\v()\v";
}
```

My reasoning is that those strings should be rejected because the specification says that the reverse solidus (`\`) is not allowed in the delimiter sequence.
https://eel.is/c++draft/lex.string#nt:d-char

> any member of the basic character set except:
>  U+0020 space, U+0028 left parenthesis, U+0029 right parenthesis, U+005c reverse solidus,
>  U+0009 character tabulation, U+000b line tabulation, U+000c form feed, and new-line

Would you please check and fix if you agree with this analysis?

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