[clang] [flang] [flang][Driver] Support -fdiagnostics-color (PR #109210)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> Is `-fdiagnostics-color=auto` too host-environment dependent to test?

I am not sure how I could reliably test it across platforms. One way to do it on Unix could be 

```
$ flang -### -diagnostics-color=auto %s | grep -Eqv '"-fc1".+"-fcolor-diagnostics"'
```

The pipe to grep would turn colors off. 

On the other hand, I don't know if  `REQUIRES: shell` ensures that the shell always has color support. If it does not guarantee that, the "normal" approach - similar to the way `-fdiagnostics-color=always` is tested - might not work. I don't have access to any non-fairly-modern-Linux'es on which I could test this either.

https://github.com/llvm/llvm-project/pull/109210