ada: Use std::string in print_dynamic_range_bound

Replace this usage of GROW_VECT with an std::string.  I don't think
there's a reason for this variable to be static, other than it was
cumbersome to manage its lifetime (i.e. use a cleanup) before.

Tested by comparing the gdb.ada/*.exp test results before and after the
patch.

gdb/ChangeLog:

	* ada-typeprint.c (print_dynamic_range_bound): Use std::string.