[flang] Make include guards consistent.

The include guard symbol is `FORTRAN_$dir_$file_H_` where $dir is the
subdirectory of `lib` and $file is the basename of the header file.
Those names are mapped to uppercase and hyphens are replaced by
underscores.

Original-commit: flang-compiler/f18@ac9c0e7106c898479cf35086861a711ab7c0210a
Reviewed-on: https://github.com/flang-compiler/f18/pull/75