[clang] [clang][scan-build] Treat --use-cc and --use-c++ as shell commands (PR #131932)

[Florian Ragwitz via cfe-commits]

rafl wrote:

Thanks for having another look @balazs-benics-sonarsource.

While we wait for a potential second opinion, I was curious about what you think of the current behaviour of silently defaulting to `$Default{CC,CXX}Compiler` if `CCC_{CC,CXX}` isn't executable and can't be resolved to an executable within `PATH`. I originally found this surprising when `CC="ccache gcc"` would be effectively ignored. Would it be worth changing this while we're at it?

Would you have any preference between these options?

1. keeping it as it is
2. warning when ignoring an option and falling back to a default
3. providing an obvious error

My personal preference is option 3. The only downside to this which I see is potentially breaking some existing usage, but it'd do so in an obvious and easy to fix way way. I'm not sure if that's acceptable in the context of this project.

Option 2 would be my second choice, with Option 1 being the least preferable.

https://github.com/llvm/llvm-project/pull/131932