[Ada] split data unpacking code out of ada_value_primitive_packed_val.

[Ada] split data unpacking code out of ada_value_primitive_packed_val.

This patch is just preparation work which splits the function
ada_value_primitive_packed_val into two function: one which unpacks
the data, and the other which now uses it to implement
ada_value_primitive_packed_val.

This simplifies a bit ada_value_primitive_packed_val, but will also
allow us to use the new function to unpack data without actually creating
a struct value as a result.

gdb/ChangeLog:

        * ada-lang.c (ada_unpack_from_contents): New function,
        extracted from ada_value_primitive_packed_val.
        (ada_value_primitive_packed_val): Replace extracted out code
        by call to ada_unpack_from_contents.