[flang] Put ISO_Fortran_binding.h where it can be easily used (#70129)

The update stems from the discussion in

https://discourse.llvm.org/t/adding-flang-specific-header-files-to-clang/72442

This is my third attempt at this. My second attempt was in pull request
#69121.

This is my second attempt at this. My first attempt was in pull request
#68756.

This pull request has three changes from the second one:
- I put the test into the Driver directory rather than Examples so that
it would get run without require the define LLVM_BUILD_EXAMPLES.
- When installing ISO_Fortran_binding.h, I changed the location where it
was installed from.
- I changed the test so that it would work when flang was built with
shared libraries.

Here's the information from my previous attempts:

I decided to put ISO_Fortran_binding.h in a place where it would be
accessible with the include: "#include<ISO_Fortran_binding.h>" rather
than "#include<fortran/ISO_Fortran_binding.h>" because this is what
gfortran im...