openmp: Fix ICE in gimplify_omp_affinity [PR103643]

After the PR90030 patch, which removes the universal casting of all Fortran
array pointers to 'c_char*', a Fortran descriptor based array passed into an
affinity() clause now looks like:

-     #pragma omp task private(i) shared(b) affinity(*(c_char *) a.data)
+     #pragma omp task private(i) shared(b) affinity(*(integer(kind=4)[0:] * restrict) a.data)

The 'integer(kind=4)[0:]' incomplete type appears to be causing ICE during
gimplify_expr() due to 'is_gimple_val, fb_rvalue'. The ICE appears to be fixed
just by adjusting to 'is_gimple_lvalue, fb_lvalue'. Considering the use of the
affinity() clause, which should be specifying the location of a particular
object in memory, this probably makes sense.

gcc/ChangeLog:

	PR middle-end/103643

	* gimplify.c (gimplify_omp_affinity): Adjust gimplify_expr of entire
	OMP_CLAUSE_DECL to use 'is_gimple_lvalue, fb_lvalue'

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/pr103643.f90: New test.