[llvm] [MC,ELF] Emit warning if a string constant contains newline char. (PR #98060)

[Dmitriy Chestnykh via llvm-commits]

chestnykh wrote:

> 1. Printing text to stdout is not the correct way to report warnings, it should be done via source manager (and it is likely there are helper functions).
> 
>     2. AsmLexer is a lexer, not parser. It does not know the context the lexed string is used. In some contexts the warning may not be desired. The warning, if needed, should be emitted by a parser that knows full context, e.g. in `AsmParser::parseDirectiveAscii`.

I reworked

https://github.com/llvm/llvm-project/pull/98060