[clang] [clang][analyzer] Delay checking the model-path (PR #150133)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

While this change looks good to me now this file system related validations will be scattered all over the codebase. I don't have a strong opinion but wondering if having a validation step right after the command line parsing is done would be better. 

Delaying some of these validations for too long can make them more lazy. E.g., if someone appends some command line flags to only do preprocessing, we might end up not doing these validations at all. There are some pros and cons to this. Clang becomes more lenient about mistakes in the command line flags but this might bite users later. When they remove/add an unrelated flag new diagnostics might pop up. 

https://github.com/llvm/llvm-project/pull/150133