gdb/fortran: Correct the lval type for array elements of internal vars

Since this commit:

  commit a5c641b5
  Date:   Thu Oct 8 16:45:59 2020 +0100

      gdb/fortran: Add support for Fortran array slices at the GDB prompt

A bug was introduced into GDB.  Consider this Fortan array:

  integer, dimension (1:10) :: array
  array = 1

Now inside GDB:

  (gdb) set $var = array
  (gdb) set $var(1) = 2
  Left operand of assignment is not an lvalue.

The problem is that the new code for slicing Fortran arrays now does
not set the lval type correctly for arrays that are not in memory.
This is easily fixed by making use of value_from_component.

After this the above example behaves as you'd expect.

gdb/ChangeLog:

	* f-lang.c (fortran_value_subarray): Call value_from_component.

gdb/testsuite/ChangeLog:

	* gdb.fortran/intvar-array.exp: New file.
	* gdb.fortran/intvar-array.f90: New file.