[Clang][C++23] P2071 Named universal character escapes

Implements [[ https://wg21.link/p2071r1  | P2071 Named Universal Character Escapes ]] - as an extension in all language mode, the patch  not warn in c++23 mode will be done later once this paper is plenary approved (in July).

We add

 * A code generator that transforms `UnicodeData.txt` and `NameAliases.txt` to a space efficient data structure that can be queried in `O(NameLength)`
 * A set of functions in `Unicode.h` to query that data, including

   * A function to find an exact match of a given Unicode character name
   * A function to perform a loose (ignoring case, space, underscore, medial hyphen) matching
   * A function returning the best matching codepoint for a given string per edit distance

 * Support of `\N{}` escape sequences in String and character Literals, with loose and typos diagnostics/fixits
 * Support of `\N{}` as UCN with loose matching diagnostics/fixits.

Loose matching is considered an error to match closely the semantics of P2071.

The generated data contributes to 280kB of data to the binaries.

`UnicodeData.txt` and `NameAliases.txt`  are not committed to the repository in this patch, and regenerating the data is a manual process.

Reviewed By: tahonermann

Differential Revision: https://reviews.llvm.org/D123064

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