gdb: add operator+= and operator+ overload for std::string

gdb: add operator+= and operator+ overload for std::string

This commit adds operator+= and operator+ overloads for adding
gdb::unique_xmalloc_ptr<char> to a std::string.  I could only find 3
places in GDB where this was useful right now, and these all make use
of operator+=.

I've also added a self test for gdb::unique_xmalloc_ptr<char>, which
makes use of both operator+= and operator+, so they are both getting
used/tested.

There should be no user visible changes after this commit, except when
running 'maint selftest', where the new self test is visible.