[flang] Fix ISO_Fortran_binding.h to work better with C++ code
This stems from working on LANL's "dopey" project --
https://github.com/lanl/dopey.
That project contains C++ code which includes the header file
"ISO_Fortran_binding.h". The dopey code wraps that include with an
'extern "C"' clause since the standard (18.5.1, paragraph 1) says that
the file" shall contain C structure definitions, typedef declarations,
...". But the clang++ compiler emits error messages objecting to the
fact that ISO_Fortran_binding.h contains templates.
This change fixes that by preceding the problematic code in
ISO_Fortran_binding.h with language linkage clauses that specify that
they contain C++ code rather than C code.
In the accompanying test, I needed to account for the fact that some
people build the compiler by doing a `make check-flang`. In this case,
the clang compiler which is required by the test will not be built.
Here's an example of a C++ program that shows the problem:
```
extern "C" {
#include "ISO_Fortran_binding.h"
}
int main() {
return 0;
}
```
