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.
820ed8af6a