Use unique_xmalloc_ptr<char> when demangling

Use unique_xmalloc_ptr<char> when demangling

I noticed that some methods in language_defn could use
unique_xmalloc_ptr<char> rather than a plain 'char *'.  This patch
implements this change, fixing up the fallout and changing
gdb_demangle to also return this type.  In one spot, std::string is
used to simplify some related code, and in another, an auto_obstack is
used to avoid manual management.

Regression tested on x86-64 Fedora 34.