[Bug c++/64608] Raw string delimiter fails to allow @ or $

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64608

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But @ and $ aren't part of the basic source character set. See 2.3
[lex.charset].