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

[Jan Schultke via cfe-commits]

eisenwave wrote:

> C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\llvm-project\clang\unittests\Basic\DiagnosticTest.cpp(454): error C2015: too many characters in constant

What is that even supposed to mean?!

https://github.com/llvm/llvm-project/blob/a7ee75a8074ab422d30d31196644564e3c271654/clang/unittests/Basic/DiagnosticTest.cpp#L452-L456

It's taking issue with the line containing the clown emoji apparently. Is this some kind of MSVC compiler bug or the result of weird compiler flags?

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