Fortran: Fix missing substring ref for allocatable saved vars [PR120483]

Fortran: Fix missing substring ref for allocatable saved vars [PR120483]

	Compute a substring ref on an allocatable static character array
	using pointer arithmetic.  Using an array type corrupts type
	layouting and crashes omp generation.

	PR fortran/120483

gcc/fortran/ChangeLog:

	* trans-expr.cc (gfc_conv_substring): Use pointer arithmetic on
	static allocatable char arrays.

gcc/testsuite/ChangeLog:

	* gfortran.dg/save_8.f90: New test.

(cherry picked from commit 5c9bdfd2748b8159856a37404ab7b34d977242ce)