Fix memory leak in create_ada_exception_catchpoint

Fix memory leak in create_ada_exception_catchpoint

Phillipe noticed that create_ada_exception_catchpoint was not freeing
the "addr_string" memory:

==14141== 114 bytes in 4 blocks are definitely lost in loss record 1,054 of 3,424
==14141==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==14141==    by 0x405107: xmalloc (common-utils.c:44)
==14141==    by 0x7563F9: xstrdup (xstrdup.c:34)
==14141==    by 0x381B21: ada_exception_sal (ada-lang.c:13217)
==14141==    by 0x381B21: create_ada_exception_catchpoint(gdbarch*, ada_exception_catchpoint_kind, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int, int, int) (ada-lang.c:13251)
==14141==    by 0x3820A8: catch_ada_exception_command(char const*, int, cmd_list_element*) (ada-lang.c:13285)
==14141==    by 0x3F4828: cmd_func(cmd_list_element*, char const*, int) (cli-decode.c:1892)

This patch fixes the problem by changing ada_exception_sal to return a
std::string via its out parameter.

gdb/ChangeLog
2019-02-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	    Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_exception_sal): Change addr_string to a
	std::string.
	(create_ada_exception_catchpoint): Update.