Make ada_decode not use a static buffer

Make ada_decode not use a static buffer

This makes it safer to use in general, and also allows using it on a
background thread in the future.

Inspired by tromey's patch at:
https://github.com/tromey/gdb/commit/1226cbdfa436297a5dec054d94592c45891afa93
(however, implemented in a different way)

gdb/ChangeLog:

2019-09-23  Christian Biesinger  <cbiesinger@google.com>

	* ada-exp.y (write_object_remaining): Update.
	* ada-lang.c (ada_decode): Return a std::string instead of a char*
	and eliminate the static buffer.
	(ada_decode_symbol): Update.
	(ada_la_decode): Update.
	(ada_sniff_from_mangled_name): Update.
	(is_valid_name_for_wild_match): Update.
	(ada_lookup_name_info::matches): Update and simplify.
	(name_matches_regex): Update.
	(ada_add_global_exceptions): Update.
	* ada-lang.h (ada_decode): Update signature.
	* ada-varobj.c (ada_varobj_describe_simple_array_child): Update.
	* dwarf-index-write.c (debug_names::insert): Update.