Extract string-printing out of ada_val_print_array

Extract string-printing out of ada_val_print_array

This patch creates a new function called "ada_val_print_string"
whose code is directly extracted out of ada_val_print_array.
The extracted code is then replaced by a call to this new function,
followed by a "return". The return avoids the need for an "else"
branch, with the associated block nesting. The latter is not really
terrible in this case, but it seems more readable this way.

gdb/ChangeLog:

        * ada-valprint.c (ada_val_print_string): New function,
        extracted from ada_val_print_array.
        (ada_val_print_array): Replace extracted code by call
        to ada_val_print_string followed by a return.  Move
        "else" branch to the function's top block.