[clang] bad error message on incorrect string literal #18079 (PR #81670)

[Jon Roelofs via cfe-commits]

jroelofs wrote:

> I am new to this project. Please let me know if any files need to be added.
> 
> I don't believe any testing or test suite changes are required for this change.

This particular change will break this usage of that diagnostic, which expects that format argument:

https://github.com/llvm/llvm-project/blob/e06f3522cc55cec60084a1278109ab236ef7a3ee/clang/lib/Lex/Lexer.cpp#L2274-L2275

The change needs to account for cases where the character should still be printed, and when it isn't printable or would have strange formatting.  And then it should be accompanied by a test that verifies the new behavior.

https://github.com/llvm/llvm-project/pull/81670