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.

gdbsupport/gdb_unique_ptr.h

@@ -21,6 +21,7 @@
 #define COMMON_GDB_UNIQUE_PTR_H
 
 #include <memory>
+#include <string>
 #include "gdbsupport/gdb-xfree.h"
 
 namespace gdb
@@ -74,4 +75,22 @@ make_unique_xstrndup (const char *str, size_t n)
   return gdb::unique_xmalloc_ptr<char> (xstrndup (str, n));
 }
 
+/* An overload of operator+= fo adding gdb::unique_xmalloc_ptr<char> to a
+   std::string.  */
+
+static inline std::string &
+operator+= (std::string &lhs, const gdb::unique_xmalloc_ptr<char> &rhs)
+{
+  return lhs += rhs.get ();
+}
+
+/* An overload of operator+ for adding gdb::unique_xmalloc_ptr<char> to a
+   std::string.  */
+
+static inline std::string
+operator+ (const std::string &lhs, const gdb::unique_xmalloc_ptr<char> &rhs)
+{
+  return lhs + rhs.get ();
+}
+
 #endif /* COMMON_GDB_UNIQUE_PTR_H */