[clang] [clang] Improve diagnostics for invalid named-universal-characters (PR #206326)

[Jan Schultke via cfe-commits]

eisenwave wrote:

Some sample output:
```
~/projects/llvm-project/build-llvm at better-ucn-diagnostics$ bin/clang++ ../clang/test/CXX/drs/bad.cpp
../clang/test/CXX/drs/bad.cpp:1:14: error: 'SS?' is not a valid Unicode character name
    1 | char c = '\N{SS?}';
      |              ^~~
../clang/test/CXX/drs/bad.cpp:1:16: note: character '?' U+003F cannot appear in a Unicode character name
    1 | char c = '\N{SS?}';
      |                ^
1 error generated.
```


https://github.com/llvm/llvm-project/pull/206326