[Clang] Correctly handle $, @, and ` when represented as UCN

This covers
 * P2558R2 (C++, wg21.link/P2558)
 * N2701 (C, https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2701.htm)
 * N3124 (C, https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3124.pdf)

This patch
 * Disallow representing $ as a UCN in all language mode, which did not
   properly work (see GH62133), and which in made ill-formed in
   C++ and C by P2558 and N3124 respectively
 * Allow a UCN for any character in C2X, in string and character
   literals

Fixes #62133

Reviewed By: #clang-language-wg, tahonermann

Differential Revision: https://reviews.llvm.org/D153621