The XL Fortran compiler's `-qmoddir=` flag sets the module output
directory but does not add the directory to the search path for using
modules.  This is inconsistent with other compilers like the GNU Fortran
compiler's `-J` flag that does both.  In order to make these consistent,
add the module output directory with a `-I` flag on the XL Fortran
compiler so that it will be searched when using modules too.

This fixes our `FortranModules` test's coverage of submodules on
Ninja + XL.  That test places module files in a subdirectory that with
Ninja is not the current working directory when the compiler runs.

Fixes: #20400