[clang] cc1: Report an error for multiple actions unless separated by -main-file-name (PR #91140)

[Justin Bogner via cfe-commits]

bogner wrote:

I don't really understand the rationale for this, and it's kind of annoying. Most of the compiler's flags behave in the "last one wins" fashion (such as `-O2` and `-O0`) and it's always been convenient to add the flag you want at the end. Why treat action flags any differently? Also, even if this is worthwhile for some reason I haven't considered, why is it an error rather than a warning?

https://github.com/llvm/llvm-project/pull/91140