[flang] Make ISO_Fortran_binding.h a standalone header again.

This implements the proposal from
https://discourse.llvm.org/t/adding-flang-specific-header-files-to-clang/72442/6
Since ISO_Fortran_binding.h is supposed to be included from users'
C/C++ codes, it would better have no dependencies on other header
files.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D158549