Respect `set print array-indexes' with Fortran arrays

Add `set print array-indexes' handling for Fortran arrays.  Currently
the setting is ignored and indices are never shown.

Keep track of the most recent index handled so that any outstanding
repeated elements printed when the limit set by `set print elements' is
hit have the correct index shown.

Output now looks like:

(gdb) set print array-indexes on
(gdb) print array_1d
$1 = ((-2) = 1, (-1) = 1, (0) = 1, (1) = 1, (2) = 1)
(gdb) set print repeats 4
(gdb) set print elements 12
(gdb) print array_2d
$2 = ((-2) = ((-2) = 2, <repeats 5 times>) (-1) = ((-2) = 2, <repeats 5 times>) (0) = ((-2) = 2, (-1) = 2, ...) ...)
(gdb)

for a 5-element vector and a 5 by 5 array filled with the value of 2.