[clang] [flang] [Flang][Driver] Add -print-resource-dir command line flag to emit Flang's resource directory (PR #90886)

[Michael Klemm via cfe-commits]

mjklemm wrote:

> What's the definition of "resource dir" for Fortran?

I'd like to at least have it point to where the MODULE files live.

When I look at what clang emits, then Flang's resource directory should rather point to the place, where Flang has its `lib` and `include` directories.  So, one level above of what I have done in this PR right now.

For reference, Clang outputs this:

```
$ which clang
clang is /usr/bin/clang
$ clang -print-resource-dir
/usr/lib/llvm-14/lib/clang/14.0.0
$ ls /usr/lib/llvm-14/lib/clang/14.0.0
total 36
24 include/   4 lib/   4 share/   4 README.txt
```

At some point, I guess Flang might also have to switch to a scheme that includes version numbers to differentiate different resource directories, so that multiple different versions of Flang can coexist.  But that's for a later PR.

https://github.com/llvm/llvm-project/pull/90886